Remove dead code

- wire.with_figure: the device uses dataclasses.replace directly.
- DeviceConnected/DeviceDisconnected: emitted, never subscribed to, and a duplicate
  of ConnectionChanged(target="firmware").
- ScreenService.toggle(): never called.
- GET /api/lirc (LircConfigOut, api.lircConfig): no front-end reads it.
- POST /api/volume's delta_percent (api.nudgeVolume): no front-end sends it.
- LibraryCache.shrink_stored_covers: a one-off migration that re-read every cover
  header on every scan. _INDEX_VERSION 7 rebuilds any cache that could still need it.
- Firmware: the MFRC522 library and its unused globals (lib/nfc/rc522.c is the
  driver in use), and mfrc_version.h, a reader test sketch nothing included.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

python-backend/README.md

@@ -220,7 +220,7 @@ copied to the device.
 | `WS /api/ws` | Push only. A snapshot on connect, then a frame per change, plus the position at 2 Hz while playing, and `clock_toggle` when the remote's Guide key asks for the clock. |
 | `POST /api/play` | `{album_id, track_index?}` |
 | `POST /api/resume` `/pause` `/next` `/previous` `/seek` | transport |
-| `POST /api/volume` | `{percent}`, `{delta_percent}`, or `{steps}` - the IR remote's step (`volume_increment` device units) per step |
+| `POST /api/volume` | `{percent}` or `{steps}` - the IR remote's step (`volume_increment` device units) per step |
 | `POST /api/remote/press/{digit}` | what the IR remote's number key does - the keyboard's digits |
 | `GET` `PUT /api/remote/mapping` | number key -> album (optionally from one `track_index`) or podcast show |
 | `POST /api/screen/toggle` `/screen/activity` | the kiosk's Del key; its key presses and taps, holding off the idle screen-off |