feat: DM epoch fix, federation relay, and mDNS mesh discovery

- schema: createChannel returns wasNew :Bool to elect the MLS initiator
  unambiguously; prevents duplicate group creation on concurrent /dm calls
- core: group helpers for epoch tracking and key-package lifecycle
- server: federation subsystem — mTLS QUIC server-to-server relay with
  Cap'n Proto RPC; enqueue/batchEnqueue relay unknown recipients to their
  home domain via FederationClient
- server: mDNS _quicproquo._udp.local. service announcement on startup
- server: storage + sql_store — identity_exists, peek/ack, federation
  home-server lookup helpers
- client: /mesh peers REPL command (mDNS discovery, feature = "mesh")
- client: MeshDiscovery — background mDNS browse with ServiceDaemon
- client: was_new=false path in cmd_dm waits for peer Welcome instead of
  creating a duplicate initiator group
- p2p: fix ALPN from quicnprotochat/p2p/1 → quicproquo/p2p/1
- workspace: re-include quicproquo-p2p in members

crates/quicproquo-server/Cargo.toml

@@ -56,5 +56,8 @@ toml          = { version = "0.8" }
 metrics                    = "0.22"
 metrics-exporter-prometheus = "0.15"
 
+# mDNS service announcement for local mesh / Freifunk node discovery.
+mdns-sd = "0.12"
+
 [dev-dependencies]
 tempfile = "3"