fix: unbounded essence farming, WS crash, and two economy races

Unbounded essence/item farming: every other reward trigger (summon,
question, fragment) had both a per-user and per-IP limiter, but
ritual_start and judgment had none at all — and judgment has no
"already resolved" state either. A scripted client could replay
{"type":"judgment","verdict":"cross_over"} in a tight loop and mint
CROSS_OVER_ESSENCE (25) plus a 20% item roll every iteration, forever.
Same for ritual_start -> 4x ritual_step. Added ritual/judgment limiters
in both flavors, matching the existing pattern.

WS session crash: _handle_question did `if state.entity is None:
await _handle_summon(state)` then `assert state.entity is not None`.
_handle_summon returns early *without* setting state.entity when the
seeker is rate-limited, so the assert fired unhandled — and the message
loop only catches WebSocketDisconnect, so it killed the whole connection.
Reachable with no malice: click summon a few times impatiently, then ask a
question. Now returns cleanly (the rate_limited frame was already sent).

Essence double-spend: purchase_unlock() deliberately uses SELECT ... FOR
UPDATE to serialize concurrent purchases, but the three credit_essence
call sites in ws.py did an unlocked db.get() read-modify-write. An
unlocked read doesn't block on a row lock, so a reward computed from a
pre-purchase balance could be written after the purchase committed,
silently reverting the deduction — user keeps the unlock and the essence.
All three now lock the row the same way.

Entity mint collision: _summon does a racy check-then-insert against
Entity.signature and Entity.name, both DB-unique, with no IntegrityError
handling — a concurrent mint of the same signature crashed the session.
Forceable by a user with two accounts (anomaly frequency/magnitude are
client-controlled), and plausible without malice in wire mode, where
sample_network() reads host-wide /proc/net/dev counters so two idle
sessions genuinely measure the same traffic. Now retries once, which
re-runs the match against whatever the winner committed.

Also added a unique constraint on unlocks(user_id, unlock_key) as
defense-in-depth, with an idempotent catalog-guarded migration.

221 backend tests pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/app/ws.py

@@ -29,6 +29,7 @@ from pathlib import Path
 
 from fastapi import APIRouter, WebSocket, WebSocketDisconnect
 from sqlalchemy import select
+from sqlalchemy.exc import IntegrityError
 
 from app import judgment
 from app.config import settings
@@ -68,6 +69,15 @@ fragment_limiter = RateLimiter(max_requests=30, window_seconds=60)
 question_limiter = RateLimiter(max_requests=6, window_seconds=60)
 summon_limiter = RateLimiter(max_requests=4, window_seconds=60)
 
+# ritual_start/judgment don't call the LLM, but each one is an essence/favor/
+# item-drop reward trigger point same as summon/question — unlike those,
+# they previously had no limiter at all, which meant a scripted client could
+# credit itself unbounded essence by simply replaying "judgment" (or
+# ritual_start -> 4x ritual_step) in a tight loop. These bound that to the
+# same modest, human-plausible cadence as the other reward triggers.
+ritual_limiter = RateLimiter(max_requests=6, window_seconds=60)
+judgment_limiter = RateLimiter(max_requests=10, window_seconds=60)
+
 # Per-IP limiters for the same trigger points (spec §5). Ollama is a shared,
 # single-instance, CPU-only resource — per-account limits alone don't stop
 # one compromised/scripted account from hammering it across many source IPs,
@@ -77,6 +87,8 @@ summon_limiter = RateLimiter(max_requests=4, window_seconds=60)
 fragment_ip_limiter = RateLimiter(max_requests=60, window_seconds=60)
 question_ip_limiter = RateLimiter(max_requests=12, window_seconds=60)
 summon_ip_limiter = RateLimiter(max_requests=8, window_seconds=60)
+ritual_ip_limiter = RateLimiter(max_requests=12, window_seconds=60)
+judgment_ip_limiter = RateLimiter(max_requests=20, window_seconds=60)
 
 AUDIO_DIR = Path(settings.data_dir) / "audio"
 
@@ -287,55 +299,73 @@ async def _summon(state: SeanceState, channel: str) -> tuple[Entity, bool]:
         str(state.session_id)
     )
 
-    async with session_maker() as db:
-        entity = await db.scalar(
-            select(Entity).where(Entity.signature == signature, Entity.at_peace.is_(False))
-        )
-        is_new = entity is None
+    # Two concurrent sessions can race to mint the same signature (this is
+    # the whole point of the match-or-mint check above being racy across
+    # connections), or two mints can independently decide on the same
+    # "next available" name via _unique_entity_name's read-then-decide
+    # check — either raises IntegrityError on commit against Entity's
+    # unique(signature)/unique(name) constraints. Retrying re-runs the
+    # match against what the winning transaction just committed, so the
+    # loser finds and reuses that row instead of crashing the session.
+    last_error: IntegrityError | None = None
+    for _attempt in range(2):
+        async with session_maker() as db:
+            try:
+                entity = await db.scalar(
+                    select(Entity).where(Entity.signature == signature, Entity.at_peace.is_(False))
+                )
+                is_new = entity is None
 
-        if is_new:
-            mint_signature = signature
-            retired = await db.scalar(select(Entity).where(Entity.signature == signature))
-            if retired is not None:
-                mint_signature = f"{signature}:{uuid.uuid4().hex[:8]}"
+                if is_new:
+                    mint_signature = signature
+                    retired = await db.scalar(select(Entity).where(Entity.signature == signature))
+                    if retired is not None:
+                        mint_signature = f"{signature}:{uuid.uuid4().hex[:8]}"
 
-            profile = await spirit_service.mint_profile(
-                mint_signature, channel, state.anomalies, state.language
-            )
-            discoverer = await db.get(User, state.user_id)
-            favor = discoverer.favor if discoverer is not None else 0.0
-            entity = Entity(
-                name=await _unique_entity_name(db, profile["name"]),
-                epithet=profile["epithet"],
-                persona=profile["persona"],
-                rarity_tier=profile["rarity"],
-                signature=mint_signature,
-                voice_profile=profile["voice"],
-                visual_profile=profile["visual"],
-                sample_quotes=profile["quotes"],
-                # Workstream B: signature-seeded traits, nudged by the
-                # discovering user's favor (app.judgment.apply_favor_bias) —
-                # never derived from/fed into the persona above.
-                traits=judgment.apply_favor_bias(profile["traits"], favor),
-                discovered_by=state.user_id,
-                contact_count=1,
-            )
-            db.add(entity)
-        else:
-            entity.contact_count += 1
+                    profile = await spirit_service.mint_profile(
+                        mint_signature, channel, state.anomalies, state.language
+                    )
+                    discoverer = await db.get(User, state.user_id)
+                    favor = discoverer.favor if discoverer is not None else 0.0
+                    entity = Entity(
+                        name=await _unique_entity_name(db, profile["name"]),
+                        epithet=profile["epithet"],
+                        persona=profile["persona"],
+                        rarity_tier=profile["rarity"],
+                        signature=mint_signature,
+                        voice_profile=profile["voice"],
+                        visual_profile=profile["visual"],
+                        sample_quotes=profile["quotes"],
+                        # Workstream B: signature-seeded traits, nudged by the
+                        # discovering user's favor (app.judgment.apply_favor_bias) —
+                        # never derived from/fed into the persona above.
+                        traits=judgment.apply_favor_bias(profile["traits"], favor),
+                        discovered_by=state.user_id,
+                        contact_count=1,
+                    )
+                    db.add(entity)
+                else:
+                    entity.contact_count += 1
 
-        await db.flush()
-        session = await db.get(ContactSession, state.session_id)
-        if session is not None:
-            session.entity_id = entity.id
-        db.add(
-            EntitySighting(
-                entity_id=entity.id, session_id=state.session_id, user_id=state.user_id
-            )
-        )
-        await db.commit()
-        await db.refresh(entity)
-        return entity, is_new
+                await db.flush()
+                session = await db.get(ContactSession, state.session_id)
+                if session is not None:
+                    session.entity_id = entity.id
+                db.add(
+                    EntitySighting(
+                        entity_id=entity.id, session_id=state.session_id, user_id=state.user_id
+                    )
+                )
+                await db.commit()
+                await db.refresh(entity)
+                return entity, is_new
+            except IntegrityError as exc:
+                await db.rollback()
+                last_error = exc
+                continue
+
+    assert last_error is not None
+    raise last_error
 
 
 async def _reward_summon(state: SeanceState) -> None:
@@ -353,7 +383,10 @@ async def _reward_summon(state: SeanceState) -> None:
     rarity = state.entity.get("rarity", "common")
 
     async with session_maker() as db:
-        user = await db.get(User, state.user_id)
+        # Locked (see purchase_unlock's docstring in app/inventory.py) so
+        # this read-modify-write on essence can't race a concurrent
+        # purchase's own locked deduction and silently clobber it.
+        user = await db.scalar(select(User).where(User.id == state.user_id).with_for_update())
         if user is None:
             return
         credit_essence(user, SUMMON_ESSENCE_TRICKLE)
@@ -480,7 +513,14 @@ async def _handle_question(state: SeanceState, text: str) -> None:
 
     if state.entity is None:
         await _handle_summon(state)
-    assert state.entity is not None
+        if state.entity is None:
+            # _handle_summon() returns without setting state.entity when
+            # the seeker is rate-limited (it already sent its own
+            # "rate_limited" error frame in that case) — bail out here
+            # instead of asserting, which would raise uncaught and crash
+            # this session's whole WS message loop (only WebSocketDisconnect
+            # is caught around it in session_socket()).
+            return
 
     text = text.strip()[:500]
     await _record_event(state.session_id, "question", text=text)
@@ -582,6 +622,18 @@ RITUAL_STEPS_REQUIRED = 4
 async def _handle_ritual_start(state: SeanceState) -> None:
     if state.entity is None:
         return  # no presence to focus on — frontend already gates the button
+    if not (
+        ritual_limiter.allow(str(state.user_id))
+        and ritual_ip_limiter.allow(state.client_ip)
+    ):
+        await state.send_queue.put(
+            {
+                "type": "error",
+                "code": "rate_limited",
+                "message": "The channel needs a moment to settle before it can be focused again.",
+            }
+        )
+        return
     state.ritual_steps = 0
     state.ritual_completed = False
     state.ritual_success = False
@@ -592,7 +644,8 @@ async def _reward_ritual_success(state: SeanceState) -> None:
     ritual milestone trigger point."""
     item = None
     async with session_maker() as db:
-        user = await db.get(User, state.user_id)
+        # Locked — see the same comment in _reward_summon above.
+        user = await db.scalar(select(User).where(User.id == state.user_id).with_for_update())
         if user is None:
             return
         credit_essence(user, RITUAL_SUCCESS_ESSENCE)
@@ -640,6 +693,18 @@ async def _handle_judgment(state: SeanceState, message: dict) -> None:
     verdict = message.get("verdict")
     if verdict not in judgment.VERDICTS:
         return
+    if not (
+        judgment_limiter.allow(str(state.user_id))
+        and judgment_ip_limiter.allow(state.client_ip)
+    ):
+        await state.send_queue.put(
+            {
+                "type": "error",
+                "code": "rate_limited",
+                "message": "The veil needs a moment before it can render another verdict.",
+            }
+        )
+        return
 
     traits = state.entity.get("traits", {})
     outcome = judgment.judge_verdict(
@@ -658,7 +723,9 @@ async def _handle_judgment(state: SeanceState, message: dict) -> None:
         "crossed_over",
     ):
         async with session_maker() as db:
-            user = await db.get(User, state.user_id)
+            # Locked — see the same comment in _reward_summon above; this
+            # path writes essence too, so it's exposed to the same race.
+            user = await db.scalar(select(User).where(User.id == state.user_id).with_for_update())
             if user is not None:
                 if outcome.favor_delta:
                     user.favor = judgment.clamp_favor(user.favor + outcome.favor_delta)