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>

README.md

@@ -118,6 +118,10 @@ make cover       # coverage per package
 make db-reset    # wipe the ledger; it is append-only and accumulates
 ```
 
+Measured capacity: **25,000 concurrent connections on one 4-core instance**,
+zero failures, 586MB RSS. Bet throughput is the real ceiling at ~230/sec —
+see [docs/SCALING.md](docs/SCALING.md).
+
 Coverage sits around 85%, and the tests found and pinned three real money
 bugs: a posting set that minted 18 quintillion millisatoshis by wrapping the
 zero-sum check, a crash point that overflowed negative at the rarest seed, and
@@ -155,6 +159,7 @@ Not yet built:
   exist; the node integration does not. The dev faucet stands in for now.
 - Tournaments and scheduled events
 - Operator dashboard
+- In-memory bet reservation, which is what would lift the ~230 bets/sec ceiling
 
 ## Scope