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/__init__.py

@@ -0,0 +1,53 @@
+"""The catalogue registry.
+
+Beefy Player speaks to internet radio through :class:`RadioProvider`
+implementations (see :mod:`beefy_player.providers.base`).  Each provider is
+registered here once; the app looks its provider up by slug, and ``--provider``
+on the command line is validated against :func:`available_providers`.
+
+Adding a catalogue is meant to be a small, contained change:
+
+1. write the package (``beefy_player/providers/<slug>/``) with a
+   :class:`~beefy_player.providers.base.RadioProvider` subclass,
+2. add one instance to :data:`PROVIDERS` below,
+3. that is it — the UI, the player, the spectrum and the media session do not
+   care where the stations came from.
+"""
+
+from __future__ import annotations
+
+from .base import RadioProvider
+from .somafm import SomaFMProvider
+
+__all__ = [
+    "DEFAULT_PROVIDER",
+    "PROVIDERS",
+    "RadioProvider",
+    "available_providers",
+    "get_provider",
+]
+
+#: Every catalogue the app can talk to, keyed by its slug.
+PROVIDERS: dict[str, RadioProvider] = {
+    provider.id: provider for provider in (SomaFMProvider(),)
+}
+
+#: Provider used when nothing else is configured.
+DEFAULT_PROVIDER = SomaFMProvider.id
+
+
+def available_providers() -> tuple[str, ...]:
+    """Registered provider slugs, for ``--provider`` and the config file."""
+    return tuple(PROVIDERS)
+
+
+def get_provider(provider_id: str | None = None) -> RadioProvider:
+    """Look a provider up by slug, falling back to the default one.
+
+    An unknown slug is a stale config file or a typo, not a reason to refuse to
+    start; the help screen and ``--list`` always name what is in use.
+    """
+    key = (provider_id or "").strip().lower()
+    if key in PROVIDERS:
+        return PROVIDERS[key]
+    return PROVIDERS[DEFAULT_PROVIDER]