Add a public current-round status endpoint

GET /rounds/current returns the active round's id/status, opened_at/
closes_at (derived from ROUND_DURATION_SECONDS), participant count and
jackpot (participant_count * bet_amount_sats). No active round still
returns bet_amount_sats so clients can render a fixed-entry hint either
way.

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

app/main.py

@@ -16,6 +16,7 @@ import app.withdrawals.confirmation  # noqa: F401  (registers the "withdrawal" c
 from app.api.routes.admin import router as admin_router
 from app.api.routes.bets import router as bets_router
 from app.api.routes.qr import router as qr_router
+from app.api.routes.rounds import router as rounds_router
 from app.api.routes.users import router as users_router
 from app.api.routes.withdrawals import router as withdrawals_router
 from app.auth.routes import router as auth_router
@@ -67,6 +68,7 @@ app.include_router(bets_router)
 app.include_router(withdrawals_router)
 app.include_router(admin_router)
 app.include_router(qr_router)
+app.include_router(rounds_router)
 
 
 @app.exception_handler(Exception)