fix(server): bind UDP media to the TCP port so self-host needs one forward rule

media_port defaulted to 0 (OS-assigned) and --port only set the TCP bind_port,
so the UDP relay bound a random high port and advertised it to clients in HELLO.
Self-hosters forwarding only 8384/udp saw connect-OK-but-no-voice, contradicting
docs/deployment.md (control and media share one port). Media now follows
bind_port when media_port is unset; 0=OS-assigned survives when bind_port is also
0 so ephemeral-port tests are unaffected. Banner now reads TCP :8384 UDP :8384.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/src/server.h

@@ -18,7 +18,7 @@ struct Config {
     std::string server_name  = "VoiceCat Server";
     std::string data_dir     = "voicecat-data";
     uint16_t    bind_port    = 8384;   // 0 = let OS pick (useful for tests)
-    uint16_t    media_port   = 0;      // M2 UDP media port; 0 = OS-assigned
+    uint16_t    media_port   = 0;      // M2 UDP media; 0 = follow bind_port (or OS-pick if that's 0)
     bool        allow_guests = true;
     // Called with the actual bound TCP port once the acceptor is ready.
     std::function<void(uint16_t)> on_ready;