feat: refund abandoned rounds; full-journey and capacity tests

Fixes the money bug flagged earlier. When an instance died mid-round its
players had already been debited, so their stakes sat with the house:
balanced books, quietly robbed players. Every instance now sweeps for
unresolved rounds and refunds them.

Such a round is marked void, not settled. The schema caught this: the
reveal_is_complete constraint requires a settled round to publish its
seed, and an abandoned round has no outcome to reveal. Void is a distinct
state with its own column and a check that the two are exclusive.
Claiming happens before money moves, so concurrent reconcilers on
different instances refund exactly once.

Adds TestFullPlayerJourney: sign-in with no account, fund, scratch, bet
with an auto target, settle, verify the round independently, check the
ledger history is continuous, transfer to a friend, and confirm the books
still sum to zero. It asserts against the ledger rather than the API's
own summary.

Adds cmd/loadtest. One instance on 4 cores held 25,000 concurrent
websocket connections with zero failures at 586MB RSS, about 26KB per
connection, with the load generator competing for the same CPU.

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

migrations/0003_void_rounds.sql

@@ -0,0 +1,21 @@
+-- A round that is abandoned is not the same as a round that settled.
+--
+-- Settlement means an outcome was produced, which is why reveal_is_complete
+-- requires a settled round to publish its seed. A round whose instance died
+-- mid-flight has no outcome at all: there is nothing to reveal, and marking it
+-- settled would either violate that constraint or, worse, publish a seed for a
+-- round that never resolved.
+--
+-- Voiding is its own state: stakes are returned and the round is closed with no
+-- result.
+
+ALTER TABLE rounds ADD COLUMN voided_at TIMESTAMPTZ;
+
+COMMENT ON COLUMN rounds.voided_at IS
+    'Set when a round was abandoned and its stakes refunded. Mutually exclusive with settled_at.';
+
+ALTER TABLE rounds ADD CONSTRAINT round_not_both_settled_and_void
+    CHECK (settled_at IS NULL OR voided_at IS NULL);
+
+CREATE INDEX rounds_unresolved_idx ON rounds (opened_at)
+    WHERE settled_at IS NULL AND voided_at IS NULL;