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/task.py

@@ -7,6 +7,8 @@ from datetime import datetime
 from pydantic import BaseModel, Field
 from enum import Enum
 
+from app.schemas.base import UTCDatetimeBaseModel
+
 
 class TaskStatusEnum(str, Enum):
     """Task status enumeration"""
@@ -146,7 +148,7 @@ class TaskUpdate(BaseModel):
     result_pdf_path: Optional[str] = None
 
 
-class TaskFileResponse(BaseModel):
+class TaskFileResponse(UTCDatetimeBaseModel):
     """Task file response schema"""
     id: int
     original_name: Optional[str] = None
@@ -156,11 +158,8 @@ class TaskFileResponse(BaseModel):
     file_hash: Optional[str] = None
     created_at: datetime
 
-    class Config:
-        from_attributes = True
 
-
-class TaskResponse(BaseModel):
+class TaskResponse(UTCDatetimeBaseModel):
     """Task response schema"""
     id: int
     user_id: int
@@ -178,9 +177,6 @@ class TaskResponse(BaseModel):
     completed_at: Optional[datetime] = None
     file_deleted: bool = False
 
-    class Config:
-        from_attributes = True
-
 
 class TaskDetailResponse(TaskResponse):
     """Detailed task response with files"""