Defines the abstraction every cover-art source will implement:
- Provider — base interface (ID, DisplayName, RequiresAPIKey,
DefaultEnabled, Configure).
- AlbumCoverProvider — opt-in capability for fetching album covers
by MBID.
- ArtistArtProvider — opt-in capability for fetching artist thumb +
fanart atomically by MBID.
- TestableProvider — opt-in capability for the admin Test-Connection
button.
Sentinel ErrNotTestable / ErrProviderNotFound. ErrNotFound and
ErrTransient stay in fetcher.go for now; they move into provider.go
in the next commit when fetcher.go is deleted as part of the MBCAA
refactor.
Compile-time registry: providers register from init() in their own
file. Duplicate IDs panic at startup (compiled-in registration is a
build-time bug, not a runtime concern).
Tests cover: Register adds, duplicate panics, ProviderByID success
+ ErrProviderNotFound, capability-interface filtering via Go type
assertion.
