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/models/sms.py

@@ -33,6 +33,8 @@ class SmsJob(Base):
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=lambda: datetime.now(timezone.utc))
     sent_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
     retry_count: Mapped[int] = mapped_column(Integer, default=0, nullable=False)
+    delivered_via: Mapped[str | None] = mapped_column(String(10), nullable=True)  # "pull" | "push"
+    processing_started_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
 
     school: Mapped["School"] = relationship("School", back_populates="sms_jobs")