Implement comprehensive Tags system for todos

- Add Tag model with id, name, color, and created_at fields
- Create TodoTag association table for many-to-many relationship
- Update Todo model to include tags relationship
- Add comprehensive Tag schemas with validation
- Update Todo schemas to support tag operations via tag_ids
- Implement full Tag CRUD operations in crud/tag.py
- Create complete Tag API with all CRUD endpoints
- Add tag-todo association endpoints (add/remove tags from todos)
- Create database migration for tags and todo_tags tables
- Update all import files to include new tag components
- Integrate tag handling in todo create/update operations
- Add API routes for /tags with full CRUD functionality

The tags system allows users to categorize and organize todos
with colored labels that can be assigned and managed independently.

alembic/versions/004_add_tags.py

@@ -14,7 +14,7 @@ import sqlalchemy as sa
 
 # revision identifiers, used by Alembic.
 revision: str = "004_add_tags"
-down_revision: Union[str, None] = "003_add_project_category"
+down_revision: Union[str, None] = "003"
 branch_labels: Union[str, Sequence[str], None] = None
 depends_on: Union[str, Sequence[str], None] = None