Make a stalled draw wait observable (B-36)

_wait_for_next_block had no timeout, no log, and no audit entry: a
connection that stopped advancing the tip left a round silently frozen
in "drawing" with nothing in /admin to explain why. Log progress
periodically, write a draw_stalled audit entry past a threshold (a few
block-time multiples), and surface the wait via a new Round.drawing_started_at
column, exposed as draw_waiting_since in GET /rounds/current.

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

app/api/routes/rounds.py

@@ -8,6 +8,7 @@ from pydantic import BaseModel
 from sqlalchemy import func, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
+from app.api.timeutil import isoformat_utc
 from app.auth.dependencies import get_optional_user
 from app.db.models import RoundParticipant, User
 from app.db.session import get_session
@@ -90,6 +91,10 @@ class CurrentRoundResponse(BaseModel):
     winner_amount_sats: int | None = None
     draw_block_height: int | None = None
     draw_block_hash: str | None = None
+    # B-36: set only while status == "drawing", so the frontend can show "still
+    # waiting for a block" rather than a countdown implying a bounded wait — this
+    # phase has no timeout, only draw_animation_seconds' cosmetic minimum.
+    draw_waiting_since: str | None = None
     chain_tip_height: int | None = None
     lottery_paused: bool = False
     user_played: bool = False
@@ -168,6 +173,7 @@ async def current_round(
         winner_amount_sats=round_.winner_amount_sats,
         draw_block_height=round_.draw_block_height,
         draw_block_hash=round_.draw_block_hash,
+        draw_waiting_since=isoformat_utc(round_.drawing_started_at) if round_.status == "drawing" else None,
         chain_tip_height=chain_tip_height,
         lottery_paused=config.paused,
         user_played=user_played,