Add transaction broadcast, confirmation polling and RBF fee-bump

Shared per-user locking to serialize bet/withdrawal PSBT builds
(tx/locks.py), a confirmation poller for pending outgoing transactions,
and the timeout->fee-bump->rebroadcast loop used by bets, payouts and
withdrawals alike (tx/broadcast.py).

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

app/tx/locks.py

@@ -0,0 +1,22 @@
+import asyncio
+from contextlib import asynccontextmanager
+
+
+class UserLocks:
+    """Per-user asyncio.Lock registry, shared by PLAY and WITHDRAW so a user can
+    never have a bet-build and a withdrawal-build in flight at once (both would
+    otherwise spend from the same UTXO set on the user's dedicated address).
+
+    Single-process-only by design (an in-memory dict of asyncio.Lock) — this is an
+    accepted MVP constraint; a multi-process deployment would need a DB or Redis
+    lock instead (e.g. a Postgres advisory lock).
+    """
+
+    def __init__(self) -> None:
+        self._locks: dict[int, asyncio.Lock] = {}
+
+    @asynccontextmanager
+    async def acquire(self, user_id: int):
+        lock = self._locks.setdefault(user_id, asyncio.Lock())
+        async with lock:
+            yield