Refactor models to improve consistency and relationships

Updated `Base` import to use centralized `app.core.database.Base` for consistency. Enhanced `TimestampMixin` and `UUIDMixin` with docstrings for clarity. Fixed metadata reference in `guest_gifts` table and added relationships in `email_template`.

backend/app/models/__init__.py

@@ -2,7 +2,9 @@
 Models package initialization.
 Imports all models to ensure they're registered with SQLAlchemy.
 """
-from .base import Base, TimestampMixin, UUIDMixin
+# First import Base to avoid circular imports
+from app.core.database import Base
+from .base import TimestampMixin, UUIDMixin
 
 # Import user model
 from .user import User