feat: auto-register mumble bots on first connect

Self-register with the Mumble server on initial connection so both
derp and merlin get persistent identities (cert-bound user_id).
Skips registration on reconnect or if already registered.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/derp/mumble.py

@@ -254,6 +254,16 @@ class MumbleBot:
                 self._mumble.users.myself.deafen()
             except Exception:
                 log.exception("mumble: failed to self-deafen on connect")
+        # Self-register on first connect so the server stores the cert
+        # and treats this bot as a known user (persistent identity).
+        if self._connect_count == 1:
+            try:
+                myself = self._mumble.users.myself
+                if not myself.get("user_id"):
+                    myself.register()
+                    log.info("mumble: self-registered %s", self._username)
+            except Exception:
+                log.debug("mumble: self-register skipped (already registered?)")
         if self._loop:
             asyncio.run_coroutine_threadsafe(
                 self._notify_plugins_connected(), self._loop,