Fix Celery datetime timezone crash and per-account polling interval

- Wrap account.last_check_at with _as_utc() helper before datetime
  subtraction to fix "can't subtract offset-naive and offset-aware
  datetimes" crash that silently prevented all mail account processing
- Change beat schedule from every 5 min to every 1 min so accounts
  configured with check_interval_minutes=1 are polled as expected

Agent-Logs-Url: https://github.com/christianlouis/InboxConverge/sessions/53169783-5139-43a1-bda8-709fadd0f774

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

backend/app/workers/celery_app.py

@@ -36,7 +36,9 @@ celery_app.conf.update(
 celery_app.conf.beat_schedule = {
     "process-all-mail-accounts": {
         "task": "app.workers.tasks.process_all_enabled_accounts",
-        "schedule": crontab(minute="*/5"),  # Every 5 minutes
+        "schedule": crontab(
+            minute="*"
+        ),  # Every minute (per-account interval gates actual work)
     },
     "cleanup-old-logs": {
         "task": "app.workers.tasks.cleanup_old_logs",