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/api/routes/rounds.py

@@ -0,0 +1,50 @@
+from datetime import timedelta, timezone
+
+from fastapi import APIRouter, Depends
+from pydantic import BaseModel
+from sqlalchemy import func, select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.config import settings
+from app.db.models import RoundParticipant
+from app.db.session import get_session
+from app.rounds.config import get_round_config
+from app.rounds.service import get_active_round
+
+router = APIRouter(prefix="/rounds", tags=["rounds"])
+
+
+class CurrentRoundResponse(BaseModel):
+    round_id: int | None = None
+    status: str | None = None
+    opened_at: str | None = None
+    closes_at: str | None = None
+    participant_count: int = 0
+    bet_amount_sats: int
+    jackpot_sats: int = 0
+
+
+@router.get("/current", response_model=CurrentRoundResponse)
+async def current_round(session: AsyncSession = Depends(get_session)) -> CurrentRoundResponse:
+    config = await get_round_config(session)
+    round_ = await get_active_round(session)
+    if round_ is None:
+        await session.commit()
+        return CurrentRoundResponse(bet_amount_sats=config.bet_amount_sats)
+
+    participant_count = await session.scalar(
+        select(func.count()).select_from(RoundParticipant).where(RoundParticipant.round_id == round_.id)
+    ) or 0
+    opened_at = round_.opened_at.replace(tzinfo=timezone.utc)
+    closes_at = opened_at + timedelta(seconds=settings.round_duration_seconds)
+    await session.commit()
+
+    return CurrentRoundResponse(
+        round_id=round_.id,
+        status=round_.status,
+        opened_at=opened_at.isoformat(),
+        closes_at=closes_at.isoformat(),
+        participant_count=participant_count,
+        bet_amount_sats=config.bet_amount_sats,
+        jackpot_sats=participant_count * config.bet_amount_sats,
+    )