test: run and fix V2 API tests - 11/18 passing

Changes:
- Fixed UserResponse schema datetime serialization bug
- Fixed test_auth.py mock structure for external auth service
- Updated conftest.py to create fresh database per test
- Ran full test suite and verified results

Test Results:
✅ test_auth.py: 5/5 passing (100%)
✅ test_tasks.py: 4/6 passing (67%)
✅ test_admin.py: 2/4 passing (50%)
❌ test_integration.py: 0/3 passing (0%)

Total: 11/18 tests passing (61%)

Known Issues:
1. Fixture isolation: test_user sometimes gets admin email
2. Admin API response structure doesn't match test expectations
3. Integration tests need mock fixes

Production Bug Fixed:
- UserResponse schema now properly serializes datetime fields to ISO format strings

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

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

backend/app/schemas/auth.py

@@ -3,6 +3,7 @@ Tool_OCR - Authentication Schemas
 """
 
 from typing import Optional
+from datetime import datetime
 from pydantic import BaseModel, Field
 
 
@@ -62,9 +63,12 @@ class UserResponse(BaseModel):
     id: int
     email: str
     display_name: Optional[str] = None
-    created_at: Optional[str] = None
-    last_login: Optional[str] = None
+    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
+        }