Add server-push (SSE) notifications for round/bet/balance state changes

Frontend dashboards previously found out about state changes only on their
next poll tick (up to 15s, or 3s during a draw) — this adds a push channel
so updates land as soon as they happen instead.

- app/rounds/events.py: a small in-process pub/sub (RoundEventBroadcaster).
  The message carries no payload — it's just a "something changed, go
  refetch" signal, so it needs no auth and no knowledge of who's allowed to
  see what; personalization stays entirely in the existing REST endpoints.
- GET /rounds/stream: an SSE endpoint exposing that channel, with keep-alive
  comments so it survives idle periods behind a reverse proxy, and a
  defensive MAX_SUBSCRIBERS cap (well above the ~100 concurrent users
  expected) — past it, the endpoint returns 503 instead of opening a stream,
  and callers just keep working off polling.
- publish() calls added at every point that actually changes what a
  dashboard would want to know: new round opened, round status transitions
  (closing/drawing/paying_out/closed), a bet or withdrawal broadcast, any
  pending tx confirming (bet/withdrawal/payout), a deposit credited, and a
  new block tip arriving (the exact moment the "drawing" phase is waiting on).
- Caddyfile: excludes /rounds/stream from gzip encoding, since compression
  would buffer output and defeat the point of a live stream.

Single-process only by design for now (no cross-worker fan-out) and the
notification is a generic broadcast rather than a per-user channel — both
are deliberate scope decisions for the current ~100-user, single-container
deployment, not oversights. Polling is left fully in place as a fallback;
this is purely additive.

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

app/rounds/events.py

@@ -0,0 +1,50 @@
+import asyncio
+
+# Defensive cap on concurrent SSE subscribers. Expected load is on the order of
+# ~100 concurrent users; this is set well above that so it never engages under
+# normal use — it exists purely so a runaway/DoS-y number of open connections
+# degrades (new connections fall back to polling, see round_stream()) instead
+# of growing the in-memory subscriber set without bound. Revisit this number if
+# expected concurrency grows well past it.
+MAX_SUBSCRIBERS = 500
+
+
+class RoundEventCapacityError(Exception):
+    """Raised by subscribe() when MAX_SUBSCRIBERS is already reached."""
+
+
+class RoundEventBroadcaster:
+    """In-process pub/sub so SSE clients (GET /rounds/stream) get pushed a
+    notification the instant round/bet/balance state changes, instead of only
+    finding out on their next poll. The message carries no payload — it's just
+    a "something changed, go refetch" signal; the client re-hits the existing
+    per-user REST endpoints (/rounds/current, /users/me, ...) for the actual
+    data, so this never needs to know what changed or who's allowed to see it.
+
+    Single-process only (no cross-worker fan-out) — fine for this deployment
+    (one uvicorn process, see docker-compose.yml). A multi-worker deployment
+    would need a shared channel (e.g. Redis pub/sub) instead.
+    """
+
+    def __init__(self, max_subscribers: int = MAX_SUBSCRIBERS):
+        self._subscribers: set[asyncio.Queue] = set()
+        self.max_subscribers = max_subscribers
+
+    def subscribe(self) -> asyncio.Queue:
+        if len(self._subscribers) >= self.max_subscribers:
+            raise RoundEventCapacityError(f"already at the {self.max_subscribers}-subscriber cap")
+        queue: asyncio.Queue = asyncio.Queue(maxsize=1)
+        self._subscribers.add(queue)
+        return queue
+
+    def unsubscribe(self, queue: asyncio.Queue) -> None:
+        self._subscribers.discard(queue)
+
+    def publish(self) -> None:
+        for queue in self._subscribers:
+            if queue.full():
+                continue  # a not-yet-delivered notification already covers this one
+            queue.put_nowait(None)
+
+
+broadcaster = RoundEventBroadcaster()

app/rounds/scheduler.py

@@ -12,6 +12,7 @@ from app.electrum.listener import ElectrumListener
 from app.electrum.scripthash import address_to_scripthash
 from app.rounds.config import get_round_config
 from app.rounds.draw import draw_winner, header_hex_to_block_hash
+from app.rounds.events import broadcaster
 from app.rounds.service import open_new_round_if_needed
 from app.wallet.hd import derive_pool_key
 from app.wallet.plm_network import PLM_MAINNET
@@ -71,6 +72,7 @@ class RoundScheduler:
                 round_.status = "closing"
                 round_.closed_at = datetime.now(timezone.utc)
                 await session.commit()
+                broadcaster.publish()
 
         async with self._session_factory() as session:
             pending_count = await session.scalar(
@@ -99,6 +101,7 @@ class RoundScheduler:
                 round_.status = "closed"
                 await write_audit_log(session, "round_closed", {"participants": 0}, round_id=round_id)
                 await session.commit()
+                broadcaster.publish()
                 logger.info("round %s closed with no participants", round_id)
                 return
 
@@ -112,6 +115,7 @@ class RoundScheduler:
 
             round_.status = "drawing"
             await session.commit()
+        broadcaster.publish()
 
         tip_at_close = self._listener.tip_height
         block_height, block_hash = await self._wait_for_next_block(tip_at_close)
@@ -139,6 +143,7 @@ class RoundScheduler:
                 round_id=round_id,
             )
             await session.commit()
+        broadcaster.publish()
 
         logger.info("round %s: winner=%s pool=%s", round_id, winner_address, pool_amount)
         await self._trigger_payout(round_id)

app/rounds/service.py

@@ -5,6 +5,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.db.models import Round
 from app.rounds.config import get_round_config
+from app.rounds.events import broadcaster
 
 _ACTIVE_STATUSES = ("open", "closing", "drawing", "paying_out")
 
@@ -57,4 +58,9 @@ async def open_new_round_if_needed(session: AsyncSession) -> Round | None:
     round_ = Round(status="open")
     session.add(round_)
     await session.flush()
+    # Published pre-commit (the caller commits right after) — acceptable: this
+    # only tells subscribers "go refetch", and by the time an SSE client's
+    # refetch request actually lands, this in-process commit (microseconds
+    # away) has essentially always already happened.
+    broadcaster.publish()
     return round_