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>

api/models/coord_todo.py

@@ -60,6 +60,11 @@ class CoordTodo(Base, UUIDMixin, TimestampMixin):
         doc="Status: pending / done / cancelled"
     )
 
+    due_at: Mapped[Optional[datetime]] = mapped_column(
+        DateTime,
+        doc="When the item is due"
+    )
+
     completed_at: Mapped[Optional[datetime]] = mapped_column(
         DateTime,
         doc="When the item was marked done"