Rename musikserverwohnzimmeroben -> musicdolphin

The Pi's OS hostname was changed first (raspi-config do_hostname +
reboot), since no mediapi sets ansible_host and Ansible resolves these
by hostname via mDNS/router DNS. Host now answers as musicdolphin.local
at 192.168.178.100.

The motd file must match the live OS hostname (pi_standard_setup reads
it off the device), the host_vars filename and the pi_musicmouse config
filename must match the inventory key.

sensor_room_name/sensor_room_name_ascii stay WohnzimmerOben - they name
the physical room for MQTT topics, not the host, and renaming them would
orphan the Home Assistant history.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -10,7 +10,7 @@ Pis (audio players, sensors, music mouse, etc.) plus one home server.
   parallel audio-player Pis) and their host-specific overrides.
 - `mediapis.yml`, `working.yml`, `server.yml`, `newrpi-provisioning.yml`,
   `octopisetup.yml` — top-level playbooks. `mediapis.yml` is the canonical
-  playbook for the `mediapis` group (`musikserverwohnzimmeroben`, `kitchenpi`,
+  playbook for the `mediapis` group (`musicdolphin`, `kitchenpi`,
   `bedroompi`, `musicmouse`); the others are narrower/ad-hoc runs kept around
   for specific hosts or one-off tasks.
 - `roles/` — one role per piece of functionality (audio backends, sensors,