Add models for EmailTemplate, ActivityLog, and NotificationLog

Introduced new database models to handle email templates, activity logs, and notification logs, including relevant enums and utilities. Updated ER diagram to reflect new relationships and attributes, enhancing event tracking and notification management capabilities.

backend/app/models/event.py

@@ -1,11 +1,10 @@
-# backend/models/event.py
-from datetime import datetime, timezone
 from sqlalchemy import (
     Column, String, DateTime, Time, Boolean, Integer, ForeignKey, JSON,
     UniqueConstraint
 )
-from sqlalchemy.orm import relationship
 from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import relationship
+
 from .base import Base, TimestampMixin, UUIDMixin
 
 
@@ -59,4 +58,4 @@ class Event(Base, UUIDMixin, TimestampMixin):
     )
 
     def __repr__(self):
-        return f"<Event {self.title} ({self.event_date})>"
+        return f"<Event {self.title} ({self.event_date})>"