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>

docs/TODO.md

@@ -14,6 +14,8 @@ Comprehensive task breakdown for repository improvements and production readines
 - [x] Fixed GitOps `update-k8s-manifest` job: corrected image tag computation and `yq` patterns to use `registry.cklnet.com` (private registry) matching the actual k8s manifest image references, so SHA-pinned tags are properly applied on each deploy.
 - [x] Added GitOps auto-deployment step in `ci.yml` to update preprod k8s manifest in `k8s-cluster-state` repo.
 - [x] Fixed GitOps `update-k8s-manifest` job: added PAT availability check to skip gracefully when `GH_PAT` secret is not configured, fixing 403 "Write access to repository not granted" pipeline failure.
+- [x] Fixed Celery `TypeError: can't subtract offset-naive and offset-aware datetimes` in `process_all_enabled_accounts` — all mail accounts were silently skipped on every scheduled run.
+- [x] Changed Celery beat schedule to run `process_all_enabled_accounts` every minute so accounts configured with `check_interval_minutes = 1` are polled as expected.
 
 ## 🔴 Critical - Security (In Progress)