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/utils/logging.py

@@ -1,7 +1,7 @@
 import logging
 import threading
 from collections import defaultdict
-from datetime import datetime
+from datetime import datetime, timezone
 
 from app.database import SessionLocal
 from app.models import FileProcessingStep, ProcessingLog
@@ -106,7 +106,7 @@ def log_task_progress(task_id, step_name, status, message=None, file_id=None, de
                 .first()
             )
 
-            now = datetime.utcnow()
+            now = datetime.now(timezone.utc)
 
             if not step_record:
                 # Create new step record