feat: add peer sync worker for zone_data exchange between nodes 🔄

Adds optional peer-to-peer zone_data replication between directdnsonly
instances. Enables eventual consistency in DA Multi-Server topologies
without a shared datastore.

- InternalAPI: GET /internal/zones (list) and ?domain= (detail)
  exposes zone_data to peers via existing basic auth
- PeerSyncWorker: interval-based daemon thread that fetches zone_data
  from configured peers, storing newer entries locally; peer downtime
  is silently skipped and retried next interval
- WorkerManager: wires PeerSyncWorker alongside reconciler; exposes
  peer_syncer_alive in queue_status
- Config: peer_sync block with enabled/interval_minutes/peers[]
- Tests: 13 tests covering sync, skip-older, skip-unreachable, empty
  peer list, bad status, and missing zone_data scenarios

tests/conftest.py

@@ -37,4 +37,5 @@ def patch_connect(db_session, monkeypatch):
     _factory = lambda: db_session  # noqa: E731
     monkeypatch.setattr("directdnsonly.app.utils.connect", _factory)
     monkeypatch.setattr("directdnsonly.app.reconciler.connect", _factory)
+    monkeypatch.setattr("directdnsonly.app.peer_sync.connect", _factory)
     return db_session