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/task.py

@@ -66,7 +66,11 @@ class Task(Base):
         return f"<Task(id={self.id}, task_id='{self.task_id}', status='{self.status.value}')>"
 
     def to_dict(self):
-        """Convert task to dictionary"""
+        """Convert task to dictionary.
+
+        All datetime fields are serialized with 'Z' suffix to indicate UTC timezone.
+        This ensures proper timezone conversion in the frontend.
+        """
         return {
             "id": self.id,
             "task_id": self.task_id,
@@ -78,11 +82,11 @@ class Task(Base):
             "result_pdf_path": self.result_pdf_path,
             "error_message": self.error_message,
             "processing_time_ms": self.processing_time_ms,
-            "created_at": self.created_at.isoformat() if self.created_at else None,
-            "updated_at": self.updated_at.isoformat() if self.updated_at else None,
-            "completed_at": self.completed_at.isoformat() if self.completed_at else None,
+            "created_at": self.created_at.isoformat() + 'Z' if self.created_at else None,
+            "updated_at": self.updated_at.isoformat() + 'Z' if self.updated_at else None,
+            "completed_at": self.completed_at.isoformat() + 'Z' if self.completed_at else None,
             "file_deleted": self.file_deleted,
-            "deleted_at": self.deleted_at.isoformat() if self.deleted_at else None
+            "deleted_at": self.deleted_at.isoformat() + 'Z' if self.deleted_at else None
         }
 
 
@@ -116,7 +120,10 @@ class TaskFile(Base):
         return f"<TaskFile(id={self.id}, task_id={self.task_id}, original_name='{self.original_name}')>"
 
     def to_dict(self):
-        """Convert task file to dictionary"""
+        """Convert task file to dictionary.
+
+        All datetime fields are serialized with 'Z' suffix to indicate UTC timezone.
+        """
         return {
             "id": self.id,
             "task_id": self.task_id,
@@ -125,5 +132,5 @@ class TaskFile(Base):
             "file_size": self.file_size,
             "mime_type": self.mime_type,
             "file_hash": self.file_hash,
-            "created_at": self.created_at.isoformat() if self.created_at else None
+            "created_at": self.created_at.isoformat() + 'Z' if self.created_at else None
         }