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

directdnsonly/config/app.yml

@@ -30,6 +30,18 @@ app:
 #      password: secret
 #      ssl: true
 
+# Peer sync — exchange zone_data between directdnsonly instances
+# Enables eventual consistency without a shared datastore.
+# If a peer is offline, the sync is silently skipped and retried next interval.
+# Use the same credentials as the peer's app.auth_username / auth_password.
+#peer_sync:
+#  enabled: true
+#  interval_minutes: 15
+#  peers:
+#    - url: http://ddo-2:2222   # URL of the peer directdnsonly instance
+#      username: directdnsonly
+#      password: changeme
+
 dns:
   default_backend: bind
   backends: