feat(providers): add a pluggable station-catalogue seam

The app now holds one RadioProvider and asks it for stations, song history and stream URLs instead of importing SomaFM's HTTP client directly.

- models.py: provider-neutral Song/Station plus the shared parsers every catalogue needs (Station.provider/.uid keep ids unique once there is more than one).
- providers/base.py: the RadioProvider interface and the toolkit it needs in practice - httpx client factory, .pls/.m3u resolution with an 8h disk cache, and a default stream_candidates() strategy.
- providers/somafm/: everything SomaFM-specific (channels.json, songs/<id>.json, PLS layout, guessed icecast mirrors) behind SomaFMProvider.
- providers/__init__.py: registry, get_provider() and --provider, remembered in the config file.

Widgets render branding, splash copy and the help credits from the active provider, so a second catalogue needs no UI changes; with SomaFM selected the screen is byte-identical to before.

tests/conftest.py

@@ -2,11 +2,12 @@ from __future__ import annotations
 
 import os
 
-os.environ["SOMAFM_IMAGES"] = "0"
+os.environ["BEEFY_IMAGES"] = "0"
 
 import pytest
 
-from somafm_tui import api, config
+from beefy_player import config
+from beefy_player.providers import base
 
 
 @pytest.fixture(autouse=True)
@@ -25,6 +26,8 @@ def isolated_state(tmp_path, monkeypatch):
         ("CONFIG_PATH", config_dir / "config.json"),
         ("CHANNELS_CACHE", cache_dir / "channels.json"),
         ("PLAYLIST_CACHE", cache_dir / "playlists.json"),
+        ("ARTWORK_CACHE_DIR", cache_dir / "artwork"),
+        ("LEGACY_CONFIG_PATH", tmp_path / "legacy" / "config.json"),
     ):
         monkeypatch.setattr(config, attr, value)
 
@@ -33,6 +36,6 @@ def isolated_state(tmp_path, monkeypatch):
             path.mkdir(parents=True, exist_ok=True)
 
     monkeypatch.setattr(config, "ensure_dirs", ensure_dirs)
-    monkeypatch.setattr(api, "_PLAYLIST_CACHE", None)
+    base.reset_playlist_cache()
     ensure_dirs()
     yield