Add IR remote control (LIRC) with a number-key content mapping

Adds a TCP client for lircd's classic protocol: play/pause/next/prev/
volume/mute map to the same intents every other front-end already
emits, and number keys 0-9 play an assigned album/audiobook from the
start or a podcast show's newest episode, resolved fresh on every
press. The mapping is configured in config.yml and editable from the
frontend: a small "Taste zuweisen" button on the play screen (or the
A+digit keyboard shortcut) opens a 10-key picker to assign whatever is
currently playing.

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

python-backend/tests/test_web.py

@@ -150,7 +150,7 @@ async def test_state_starts_idle(client: Client) -> None:
     assert state["playing"] is False
     assert state["album_id"] is None
     assert state["active_figure"] is None
-    assert state["connected"] == {"firmware": True, "mqtt": False}
+    assert state["connected"] == {"firmware": True, "mqtt": False, "lirc": False}
 
 
 async def test_play_loads_the_album_and_starts_it(client: Client, sim: Simulation) -> None: