feat(coord): add due_at field to coord_todos

Migration 20260526_150000 adds nullable due_at datetime column. Model, schemas
(create/update/response), and sync.sh display updated. Sync output now shows
due:YYYY-MM-DDTHH:MM alongside any todo with a due date.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

migrations/versions/20260526_150000_coord_todos_due_at.py

@@ -0,0 +1,24 @@
+"""coord_todos add due_at column
+
+Revision ID: 20260526_150000
+Revises: 20260526_120000
+Create Date: 2026-05-26 15:00:00
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "20260526_150000"
+down_revision: Union[str, None] = "20260526_120000"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column("coord_todos", sa.Column("due_at", sa.DateTime(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("coord_todos", "due_at")