Fix Todo schema to include tags relationship

- Add tags field to Todo schema for proper serialization
- Update forward references to include Tag schema
- Ensure Todo schema includes project_id field for completeness

app/schemas/todo.py

@@ -6,6 +6,7 @@ from app.models.todo import Priority
 
 if TYPE_CHECKING:
     from app.schemas.category import Category
+    from app.schemas.project import Project
 
 
 class TodoBase(BaseModel):