feat(phase-5): on-prem sync poll hardening + stale job reclaim

Harden the Hub-side polling protocol for on-prem TapTrack agents:
- sync/poll: accept job_failed_ids (retry/fail on-prem delivery failures)
- sync/poll: deduct credits + write ledger when on-prem reports sent jobs
- sync/poll: return feature_flags (tier-based) + suspended flag in config
- sync/poll: skip job dispatch for suspended/expired schools
- sms_jobs: add delivered_via (pull|push) + processing_started_at columns
- tasks/sms: new sms.reclaim_stale_jobs task resets processing→pending if on-prem
  goes offline (jobs stuck >5 min), enabling Celery push fallback
- tasks/sms: tag Celery-sent jobs as delivered_via='push'
- worker: schedule reclaim_stale_jobs every 5 minutes
- migration: 001_phase5 adds delivered_via + processing_started_at to sms_jobs

backend/app/worker.py

@@ -24,11 +24,16 @@ celery_app.conf.update(
     timezone="Asia/Manila",
     enable_utc=True,
     beat_schedule={
-        # Process pending SMS jobs every 30 seconds
+        # Process pending SMS jobs every 30 seconds (push path via Semaphore)
         "process-sms-queue": {
             "task": "sms.process_queue",
             "schedule": 30.0,
         },
+        # Reclaim processing jobs where on-prem went offline (every 5 minutes)
+        "reclaim-stale-sms-jobs": {
+            "task": "sms.reclaim_stale_jobs",
+            "schedule": 300.0,
+        },
         # Check license expiry every day at 8am
         "check-license-expiry": {
             "task": "license.check_expiry",