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/email_template.py

@@ -1,6 +1,8 @@
 from enum import Enum
-from sqlalchemy import Column, String, Boolean, Enum as SQLEnum, Text, UniqueConstraint
+from sqlalchemy import Column, String, Boolean, Enum as SQLEnum, Text, UniqueConstraint, ForeignKey
 from sqlalchemy.dialects.postgresql import UUID, JSONB
+from sqlalchemy.orm import relationship
+
 from .base import Base, TimestampMixin, UUIDMixin