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.

beefy_player/providers/somafm/__init__.py

@@ -0,0 +1,55 @@
+"""SomaFM: the station catalogue Beefy Player was born with.
+
+Everything SomaFM-specific lives in this package — the ``channels.json``
+schema, the ``songs/<station>.json`` history, the ``.pls`` layout and the
+guessed icecast mirrors.  The rest of the app only sees
+:class:`SomaFMProvider`, which implements
+:class:`beefy_player.providers.base.RadioProvider`.
+"""
+
+from __future__ import annotations
+
+import httpx
+
+from ... import config
+from ...models import Song, Station
+from ..base import RadioProvider
+from . import api, fallback
+
+__all__ = ["SomaFMProvider"]
+
+
+class SomaFMProvider(RadioProvider):
+    id = "somafm"
+    name = "SomaFM"
+    homepage = "somafm.com"
+    brand = "SOMA FM"
+    tagline = "listener-supported radio · san francisco"
+    station_source = "somafm.com/channels.json"
+    history_source = "songs/<station>.json"
+
+    @property
+    def user_agent(self) -> str:
+        # SomaFM asks clients to identify themselves, so say who we are and
+        # where we come from.
+        return f"{config.USER_AGENT}; +https://somafm.com"
+
+    async def fetch_stations(self, client: httpx.AsyncClient) -> list[Station]:
+        return await api.fetch_channels(client)
+
+    def load_cached_stations(self) -> list[Station]:
+        return api.load_cached_channels()
+
+    def fallback_stations(self) -> list[Station]:
+        return api.parse_channels({"channels": fallback.fallback_channels()})
+
+    async def fetch_history(
+        self, client: httpx.AsyncClient, station_id: str
+    ) -> list[Song]:
+        return await api.fetch_history(client, station_id)
+
+    def load_cached_history(self, station_id: str) -> list[Song]:
+        return api.load_cached_songs(station_id)
+
+    def fallback_urls(self, station: Station) -> list[str]:
+        return api.synthetic_mirrors(station.id)