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/tasks/upload_to_google_drive.py

@@ -20,6 +20,9 @@ from app.utils import log_task_progress
 
 logger = logging.getLogger(__name__)
 
+# Google OAuth constants
+_GOOGLE_TOKEN_URL = "https://oauth2.googleapis.com/token"
+
 
 def get_drive_service_oauth():
     """
@@ -40,7 +43,7 @@ def get_drive_service_oauth():
         credentials = OAuthCredentials(
             None,  # No access token initially, will be refreshed
             refresh_token=settings.google_drive_refresh_token,
-            token_uri="https://oauth2.googleapis.com/token",
+            token_uri=_GOOGLE_TOKEN_URL,
             client_id=settings.google_drive_client_id,
             client_secret=settings.google_drive_client_secret,
             # Use only drive.file scope