fix: add UTC timezone indicator to all datetime serialization

Database stores times in UTC but serialized without timezone info,
causing frontend to misinterpret as local time. Now all datetime
fields include 'Z' suffix to indicate UTC, enabling proper timezone
conversion in the browser.

- Add UTCDatetimeBaseModel base class for Pydantic schemas
- Update model to_dict() methods to append 'Z' suffix
- Affects: tasks, users, sessions, audit logs, translations

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/models/user.py

@@ -39,12 +39,15 @@ class User(Base):
         return f"<User(id={self.id}, email='{self.email}', display_name='{self.display_name}')>"
 
     def to_dict(self):
-        """Convert user to dictionary"""
+        """Convert user to dictionary.
+
+        All datetime fields are serialized with 'Z' suffix to indicate UTC timezone.
+        """
         return {
             "id": self.id,
             "email": self.email,
             "display_name": self.display_name,
-            "created_at": self.created_at.isoformat() if self.created_at else None,
-            "last_login": self.last_login.isoformat() if self.last_login else None,
+            "created_at": self.created_at.isoformat() + 'Z' if self.created_at else None,
+            "last_login": self.last_login.isoformat() + 'Z' if self.last_login else None,
             "is_active": self.is_active
         }