feat: retry queue, backend healing, and zone_data persistence 🔁

- worker.py: third persistent retry queue with exponential backoff (30s→30m,
  max 5 attempts); failed backends tracked per-item so retries target only the
  failing nodes; zone_data stored in DB after every successful write
- Domain model: zone_data TEXT + zone_updated_at DATETIME columns; additive
  migration applied on startup so existing deployments upgrade in place
- ReconciliationWorker: Option C healing pass — checks every configured backend
  for zone presence after each reconciliation cycle and re-queues any zone
  missing from a backend using stored zone_data, enabling automatic recovery
  from prolonged backend outages without waiting for DirectAdmin to re-push
- 82 tests, all passing

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "directdnsonly"
-version = "2.2.0"
+version = "2.3.0"
 description = "DNS Management System - DirectAdmin to multiple backends"
 authors = [
     {name = "Aaron Guise",email = "aaron@guise.net.nz"}