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_library.py

@@ -128,6 +128,28 @@ async def test_podcast_episodes_are_newest_first(config_dir: Path) -> None:
     assert [a.title for a in episodes] == ["Neu", "Alt"]
 
 
+async def test_latest_episode_finds_the_newest_episode_of_a_show(config_dir: Path) -> None:
+    library = await build(config_dir)
+    latest = library.latest_episode("Wissen macht Ah")
+    assert latest is not None
+    assert latest.title == "Neu"
+
+
+async def test_latest_episode_is_none_for_an_unknown_show(config_dir: Path) -> None:
+    library = await build(config_dir)
+    assert library.latest_episode("no such show") is None
+
+
+async def test_latest_episode_ignores_series_that_are_not_episode_unit(config_dir: Path) -> None:
+    """A music/book section groups by `series` too (an audiobook's character), but it
+    is a folder-unit shelf, not an episode-unit one - `latest_episode` must not treat
+    an audiobook as if it had "episodes"."""
+    library = await build(config_dir)
+    audiobook = album_named(library, "Conni in den Bergen")
+    assert audiobook.series is not None
+    assert library.latest_episode(audiobook.series) is None
+
+
 async def test_other_sections_keep_filename_order(config_dir: Path) -> None:
     album = album_named(await build(config_dir), "Kinderparty Lieder")
     assert [track.path.name for track in album.tracks] == [