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_config.py

@@ -56,6 +56,46 @@ def test_ha_section_is_optional(config_dir: Path) -> None:
     assert load_config(write_config(config_dir, VALID_CONFIG)).general.ha is None
 
 
+def test_lirc_section_is_optional(config_dir: Path) -> None:
+    assert load_config(write_config(config_dir, VALID_CONFIG)).general.lirc is None
+
+    data = _config(lirc={"host": "musicmouse-pi.local"})
+    config = load_config(write_config(config_dir, data))
+    assert config.general.lirc is not None
+    assert config.general.lirc.port == 2222
+    assert config.general.lirc.remote_name == "Hauppauge"
+
+
+def test_remote_mapping_is_optional_and_empty_by_default(config_dir: Path) -> None:
+    assert load_config(write_config(config_dir, VALID_CONFIG)).remote == {}
+
+
+def test_remote_mapping_loads_album_and_series_slots(config_dir: Path) -> None:
+    data = copy.deepcopy(VALID_CONFIG)
+    data["remote"] = {
+        "1": {"target_kind": "album", "target": "abc123"},
+        "7": {"target_kind": "series", "target": "Wissen macht Ah"},
+    }
+    config = load_config(write_config(config_dir, data))
+    assert config.remote["1"].target_kind == "album"
+    assert config.remote["1"].target == "abc123"
+    assert config.remote["7"].target_kind == "series"
+
+
+def test_remote_mapping_rejects_an_out_of_range_digit(config_dir: Path) -> None:
+    data = copy.deepcopy(VALID_CONFIG)
+    data["remote"] = {"10": {"target_kind": "album", "target": "abc123"}}
+    message = _error(config_dir, data)
+    assert "remote" in message
+
+
+def test_remote_mapping_rejects_an_unknown_target_kind(config_dir: Path) -> None:
+    data = copy.deepcopy(VALID_CONFIG)
+    data["remote"] = {"1": {"target_kind": "playlist", "target": "abc123"}}
+    message = _error(config_dir, data)
+    assert "remote" in message
+
+
 def test_ha_device_and_scene_name_is_optional(config_dir: Path) -> None:
     data = _config(
         ha={