Add cluster replication features and membership management

- Introduced a new `ClusterManager` to handle cluster membership events and facilitate event replication across relay peers.
- Implemented HTTP endpoints for retrieving the latest serial and fetching events within a specified range.
- Enhanced event handling to process cluster membership events (Kind 39108) and update relay lists accordingly.
- Updated configuration to support cluster administrators and their management capabilities.
- Added comprehensive tests to validate the new cluster replication functionalities.
- Documented the cluster replication protocol in a new specification file.
- Bumped version to reflect these changes.

app/config/config.go

@@ -52,6 +52,7 @@ type C struct {
 	RelayAddresses      []string      `env:"ORLY_RELAY_ADDRESSES" usage:"comma-separated list of websocket addresses for this relay (e.g., wss://relay.example.com,wss://backup.example.com)"`
 	RelayPeers          []string      `env:"ORLY_RELAY_PEERS" usage:"comma-separated list of peer relay URLs for distributed synchronization (e.g., https://peer1.example.com,https://peer2.example.com)"`
 	RelayGroupAdmins    []string      `env:"ORLY_RELAY_GROUP_ADMINS" usage:"comma-separated list of npubs authorized to publish relay group configuration events"`
+	ClusterAdmins       []string      `env:"ORLY_CLUSTER_ADMINS" usage:"comma-separated list of npubs authorized to manage cluster membership"`
 	FollowListFrequency time.Duration `env:"ORLY_FOLLOW_LIST_FREQUENCY" usage:"how often to fetch admin follow lists (default: 1h)" default:"1h"`
 
 	// Blossom blob storage service level settings