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/monitor_stalled_steps.py

@@ -6,7 +6,7 @@ that have been stuck in "in_progress" state for too long and mark them as failed
 """
 
 import logging
-from datetime import datetime
+from datetime import datetime, timezone
 
 from app.celery_app import celery
 from app.database import SessionLocal
@@ -37,12 +37,12 @@ def monitor_stalled_steps():
 
             if stalled_count > 0:
                 logger.warning(
-                    f"[{datetime.utcnow().isoformat()}] "
+                    f"[{datetime.now(timezone.utc).isoformat()}] "
                     f"Recovered {stalled_count} stalled step(s). "
                     f"Marked as failed due to timeout."
                 )
             else:
-                logger.debug(f"[{datetime.utcnow().isoformat()}] No stalled steps found.")
+                logger.debug(f"[{datetime.now(timezone.utc).isoformat()}] No stalled steps found.")
 
             return {"recovered": stalled_count}