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/schemas/auth.py

@@ -6,6 +6,8 @@ from typing import Optional
 from datetime import datetime
 from pydantic import BaseModel, Field
 
+from app.schemas.base import UTCDatetimeBaseModel
+
 
 class LoginRequest(BaseModel):
     """Login request schema"""
@@ -58,7 +60,7 @@ class TokenData(BaseModel):
     session_id: Optional[int] = None
 
 
-class UserResponse(BaseModel):
+class UserResponse(UTCDatetimeBaseModel):
     """User response schema"""
     id: int
     email: str
@@ -66,9 +68,3 @@ class UserResponse(BaseModel):
     created_at: Optional[datetime] = None
     last_login: Optional[datetime] = None
     is_active: bool = True
-
-    class Config:
-        from_attributes = True
-        json_encoders = {
-            datetime: lambda v: v.isoformat() if v else None
-        }