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/migrations/versions/001_phase5_sms_job_delivery_tracking.py

@@ -0,0 +1,35 @@
+"""phase5: add delivered_via and processing_started_at to sms_jobs
+
+Revision ID: 001_phase5
+Revises:
+Create Date: 2026-03-16
+
+Adds two columns to sms_jobs to support the on-prem polling protocol:
+- delivered_via: tracks whether the job was sent via the on-prem pull agent ("pull")
+                 or directly via the Hub's Celery push path ("push")
+- processing_started_at: timestamp when the job was marked `processing` (dispatched to on-prem).
+                         Used by sms.reclaim_stale_jobs to detect and reclaim stuck jobs.
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "001_phase5"
+down_revision = None
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "sms_jobs",
+        sa.Column("delivered_via", sa.String(10), nullable=True),
+    )
+    op.add_column(
+        "sms_jobs",
+        sa.Column("processing_started_at", sa.DateTime(timezone=True), nullable=True),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("sms_jobs", "processing_started_at")
+    op.drop_column("sms_jobs", "delivered_via")