fix: blank inbox previews, two unauthenticated 500s, and the missing social tests

- Every conversation row in the inbox rendered an empty preview: the
  frontend read `last_body`, which the API has never sent. It sends
  `excerpt` plus `truncated`.
- `GET /api/codex?limit=-1` passed a negative LIMIT to Postgres -> 500.
  Now bounded by Query(ge=1, le=200) -> 422.
- Registering with a >255 char email hit the VARCHAR(255) column and
  surfaced as a 500. PATCH /api/profile already validated this; only
  /auth/register was open.

The social layer had zero tests, despite routes/messages.py citing
"tests/test_messages.py's cross-user leak tests" and routes/profile.py
claiming "privacy rules that the tests pin". Neither file existed. Both now
do: auth required on every route, email only on /api/profile/me, no id or
password_hash on any public payload, third-party thread reads empty,
payload-supplied sender inert, veiled profiles 404 and stay off the roster.

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

backend/app/routes/codex.py

@@ -3,7 +3,7 @@ contacted, shared across all users (spec §4)."""
 
 import uuid
 
-from fastapi import APIRouter, Depends, HTTPException, status
+from fastapi import APIRouter, Depends, HTTPException, Query, status
 from sqlalchemy import desc, func, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -126,7 +126,9 @@ def _entity_card(entity: Entity, discoverer: str | None) -> dict:
 async def list_codex(
     rarity: str | None = None,
     sort: str = "recent",
-    limit: int = 60,
+    # Bounded at the edge: an unvalidated `?limit=-1` reached Postgres as a
+    # negative LIMIT and returned a 500 to an unauthenticated caller.
+    limit: int = Query(default=60, ge=1, le=200),
     db: AsyncSession = Depends(get_db),
 ):
     query = select(Entity)