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/db/models.py

@@ -72,6 +72,11 @@ class Round(Base):
     status: Mapped[str] = mapped_column(String(16), default="open")
     opened_at: Mapped[datetime] = mapped_column(default=utcnow)
     closed_at: Mapped[datetime | None] = mapped_column(default=None)
+    # Set once, when status flips to "drawing" (rounds/scheduler.py:_close_and_draw).
+    # Lets both the audit log (B-36's draw_stalled entries) and GET /rounds/current
+    # (draw_waiting_since) measure how long a round has been waiting on a block,
+    # since that wait has no timeout of its own — see _wait_for_next_block.
+    drawing_started_at: Mapped[datetime | None] = mapped_column(default=None)
     draw_block_height: Mapped[int | None] = mapped_column(default=None)
     draw_block_hash: Mapped[str | None] = mapped_column(String(64), default=None)
     seed_int: Mapped[str | None] = mapped_column(String(128), default=None)