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/schemas/coord_todo.py

@@ -19,6 +19,7 @@ class CoordTodoCreate(BaseModel):
     assigned_to_machine: Optional[str] = Field(None, description="Target hostname", max_length=100)
     auto_created: bool = Field(False, description="True when Claude auto-generated this item")
     source_context: Optional[str] = Field(None, description="Why Claude auto-created this item")
+    due_at: Optional[datetime] = Field(None, description="When the item is due")
     created_by_user: str = Field(..., description="User creating the item", max_length=50)
     created_by_machine: str = Field(..., description="Hostname where the item is created", max_length=100)
 
@@ -31,6 +32,7 @@ class CoordTodoUpdate(BaseModel):
     assigned_to_user: Optional[str] = Field(None, max_length=50)
     assigned_to_machine: Optional[str] = Field(None, max_length=100)
     project_key: Optional[str] = Field(None, max_length=100)
+    due_at: Optional[datetime] = Field(None, description="When the item is due")
     completed_by: Optional[str] = Field(None, description="Session or user completing the item", max_length=100)
 
 
@@ -45,6 +47,7 @@ class CoordTodoResponse(BaseModel):
     assigned_to_machine: Optional[str]
     auto_created: bool
     source_context: Optional[str]
+    due_at: Optional[datetime]
     status: str
     completed_at: Optional[datetime]
     completed_by: Optional[str]