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/base.py

@@ -1,18 +1,20 @@
-# backend/models/base.py
-from datetime import datetime, timezone
 import uuid
-from sqlalchemy.ext.declarative import declarative_base
-from sqlalchemy import Column, DateTime, Boolean
+from datetime import datetime, timezone
+
+from sqlalchemy import Column, DateTime
 from sqlalchemy.dialects.postgresql import UUID
 
-Base = declarative_base()
+# noinspection PyUnresolvedReferences
+from app.core.database import Base
 
 
 class TimestampMixin:
+    """Mixin to add created_at and updated_at timestamps to models"""
     created_at = Column(DateTime(timezone=True), default=lambda: datetime.now(timezone.utc), nullable=False)
     updated_at = Column(DateTime(timezone=True), default=lambda: datetime.now(timezone.utc),
                         onupdate=lambda: datetime.now(timezone.utc), nullable=False)
 
 
 class UUIDMixin:
-    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    """Mixin to add UUID primary keys to models"""
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)