Number keys: the keyboard's digits work like the remote's; a key can hold one song

The IR remote's digits and the kiosk keyboard's number keys now both emit
PlayRemoteSlotRequested, and one reaction resolves the slot for either (a podcast
show to its newest episode, an album to the album). POST /api/remote/press/{digit}
is the keyboard's way in. Mid-search a digit is still part of the query.

A remote slot can start one song instead of a whole album: `track_index` on an
album slot plays from that track, then the rest of the album. Shift+A on the play
screen arms "this song" where A arms "this album", and the assign popup offers
both. A series slot cannot name a song, and an index past the album's end is
rejected when saving.

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

python-backend/tests/test_lirc.py

@@ -383,6 +383,29 @@ async def test_an_album_digit_plays_from_the_beginning(
     assert sim.player.track_index == 0
 
 
+async def test_a_song_digit_plays_from_that_song(
+    lircd: tuple[ScriptedLircd, asyncio.base_events.Server], sim: Simulation
+) -> None:
+    station, server = lircd
+    port = server.sockets[0].getsockname()[1]
+    album = next(a for a in sim.app.library.albums if a.title == "Kinderparty Lieder")
+    sim.app.config.remote = {
+        "3": RemoteSlotConfig(target_kind="album", target=album.id, track_index=1)
+    }
+
+    task = await _run_service(sim, _lirc_config(port))
+    try:
+        await station.wait_connected()
+        await station.send("0 00 BTN_3 Hauppauge")
+        await sim.bus.drain()
+    finally:
+        task.cancel()
+
+    assert sim.player.playlist is not None
+    assert sim.player.playlist.album_id == album.id
+    assert sim.player.track_index == 1
+
+
 async def test_a_series_digit_plays_the_latest_episode(
     lircd: tuple[ScriptedLircd, asyncio.base_events.Server], sim: Simulation
 ) -> None: