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/bets/service.py

@@ -8,6 +8,7 @@ from app.audit.log import write_audit_log
 from app.db.models import PendingTransaction, RoundParticipant, User, UtxoEvent
 from app.electrum.client import ElectrumClient
 from app.rounds.config import get_round_config
+from app.rounds.events import broadcaster
 from app.rounds.service import open_new_round_if_needed, round_accepts_bets
 from app.wallet.balance import recompute_balance
 from app.wallet.hd import derive_pool_address, derive_user_key
@@ -91,6 +92,7 @@ async def place_bet(session: AsyncSession, client: ElectrumClient, user: User) -
 
     await session.commit()
     await session.refresh(participant)
+    broadcaster.publish()  # participant_count/jackpot changed — nudge every dashboard to refetch
     return participant