fix: apply Phase 1-2 correctness and constant extraction from PR #273

- Replace datetime.utcnow() with datetime.now(timezone.utc) in 4 files
- Extract duplicate literals to constants in 5 files
  - models.py: "files.id" → _FILES_ID_FK
  - upload_to_email.py: "logo.png" → _LOGO_FILENAME
  - general.py: "%B %d, %Y" → _DATE_DISPLAY_FORMAT
  - files.py: "File not found" → _FILE_NOT_FOUND
  - upload_to_google_drive.py: Google token URL → _GOOGLE_TOKEN_URL

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/api/diagnostic.py

@@ -64,7 +64,7 @@ async def test_notification(request: Request):
     # Add request_time to request.state
     import datetime
 
-    request.state.request_time = datetime.datetime.utcnow().isoformat()
+    request.state.request_time = datetime.datetime.now(datetime.timezone.utc).isoformat()
     """
     Send a test notification through all configured notification channels
     """