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
-        }

backend/app/schemas/base.py

@@ -0,0 +1,38 @@
+"""
+Tool_OCR - Base Schema with UTC Datetime Serialization
+
+All datetime fields stored in the database are in UTC (using datetime.utcnow()).
+This base class ensures proper serialization with 'Z' suffix to indicate UTC,
+allowing frontend to correctly convert to local timezone.
+"""
+
+from datetime import datetime
+from typing import Any
+from pydantic import BaseModel, ConfigDict
+
+
+def serialize_datetime_utc(dt: datetime) -> str:
+    """Serialize datetime to ISO format with UTC indicator (Z suffix).
+
+    Database stores all times in UTC without timezone info (naive datetime).
+    This serializer adds the 'Z' suffix to indicate UTC, enabling proper
+    timezone conversion in the frontend.
+    """
+    if dt is None:
+        return None
+    # Ensure ISO format with 'Z' suffix for UTC
+    return dt.isoformat() + 'Z'
+
+
+class UTCDatetimeBaseModel(BaseModel):
+    """Base model that serializes all datetime fields with UTC indicator.
+
+    Use this as base class for any response schema containing datetime fields
+    to ensure consistent timezone handling across the API.
+    """
+    model_config = ConfigDict(
+        from_attributes=True,
+        json_encoders={
+            datetime: serialize_datetime_utc
+        }
+    )

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"""

backend/app/schemas/translation.py

@@ -9,6 +9,8 @@ from pydantic import BaseModel, Field
 from enum import Enum
 from dataclasses import dataclass
 
+from app.schemas.base import UTCDatetimeBaseModel
+
 
 class TranslationStatusEnum(str, Enum):
     """Translation job status enumeration"""
@@ -54,7 +56,7 @@ class TranslationProgress(BaseModel):
     percentage: float = Field(default=0.0, description="Progress percentage (0-100)")
 
 
-class TranslationStatusResponse(BaseModel):
+class TranslationStatusResponse(UTCDatetimeBaseModel):
     """Response model for translation status query"""
     task_id: str = Field(..., description="Task ID")
     status: TranslationStatusEnum = Field(..., description="Current translation status")
@@ -89,7 +91,7 @@ class TranslationStatistics(BaseModel):
     total_tokens: int = Field(default=0, description="Total API tokens used")
 
 
-class TranslationResultResponse(BaseModel):
+class TranslationResultResponse(UTCDatetimeBaseModel):
     """Response model for translation result"""
     schema_version: str = Field(default="1.0.0", description="Schema version")
     source_document: str = Field(..., description="Source document filename")
@@ -104,7 +106,7 @@ class TranslationResultResponse(BaseModel):
     )
 
 
-class TranslationListItem(BaseModel):
+class TranslationListItem(UTCDatetimeBaseModel):
     """Item in translation list response"""
     target_lang: str = Field(..., description="Target language")
     translated_at: datetime = Field(..., description="Translation timestamp")