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>

migrations/versions/9ef6a51509f7_add_drawing_started_at_to_rounds.py

@@ -0,0 +1,32 @@
+"""add drawing_started_at to rounds
+
+Revision ID: 9ef6a51509f7
+Revises: 943dbd74d983
+Create Date: 2026-07-27 12:31:09.907682
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '9ef6a51509f7'
+down_revision: Union[str, Sequence[str], None] = '943dbd74d983'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.add_column('rounds', sa.Column('drawing_started_at', sa.DateTime(), nullable=True))
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_column('rounds', 'drawing_started_at')
+    # ### end Alembic commands ###