feat(server/m7-cover-sources): Provider interface + registry

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.

internal/coverart/provider_test.go

@@ -0,0 +1,116 @@
+package coverart
+
+import (
+	"context"
+	"errors"
+	"testing"
+)
+
+// fakeProvider is a minimal Provider for registry tests. Does not
+// implement any capability interface.
+type fakeProvider struct {
+	id          string
+	display     string
+	requiresKey bool
+	defaultOn   bool
+}
+
+func (f *fakeProvider) ID() string                         { return f.id }
+func (f *fakeProvider) DisplayName() string                { return f.display }
+func (f *fakeProvider) RequiresAPIKey() bool               { return f.requiresKey }
+func (f *fakeProvider) DefaultEnabled() bool               { return f.defaultOn }
+func (f *fakeProvider) Configure(s ProviderSettings) error { return nil }
+
+// fakeAlbumProvider opts into the AlbumCoverProvider capability for
+// the capability-filter test.
+type fakeAlbumProvider struct {
+	fakeProvider
+}
+
+func (f *fakeAlbumProvider) FetchAlbumCover(ctx context.Context, mbid string) ([]byte, error) {
+	return []byte("img"), nil
+}
+
+func TestRegister_AddsProvider(t *testing.T) {
+	resetRegistryForTests()
+	defer resetRegistryForTests()
+
+	p := &fakeProvider{id: "fake1", display: "Fake One"}
+	Register(p)
+
+	all := AllProviders()
+	if len(all) != 1 {
+		t.Fatalf("AllProviders len = %d, want 1", len(all))
+	}
+	if all[0].ID() != "fake1" {
+		t.Errorf("ID = %q, want %q", all[0].ID(), "fake1")
+	}
+}
+
+func TestRegister_PanicsOnDuplicateID(t *testing.T) {
+	resetRegistryForTests()
+	defer resetRegistryForTests()
+
+	Register(&fakeProvider{id: "dup"})
+
+	defer func() {
+		if r := recover(); r == nil {
+			t.Fatal("expected panic on duplicate ID")
+		}
+	}()
+	Register(&fakeProvider{id: "dup"}) // should panic
+}
+
+func TestProviderByID_Found(t *testing.T) {
+	resetRegistryForTests()
+	defer resetRegistryForTests()
+
+	Register(&fakeProvider{id: "x", display: "X"})
+
+	p, err := ProviderByID("x")
+	if err != nil {
+		t.Fatalf("err = %v, want nil", err)
+	}
+	if p.DisplayName() != "X" {
+		t.Errorf("display = %q, want %q", p.DisplayName(), "X")
+	}
+}
+
+func TestProviderByID_NotFound(t *testing.T) {
+	resetRegistryForTests()
+	defer resetRegistryForTests()
+
+	_, err := ProviderByID("missing")
+	if !errors.Is(err, ErrProviderNotFound) {
+		t.Errorf("err = %v, want ErrProviderNotFound", err)
+	}
+}
+
+func TestCapabilityFiltering(t *testing.T) {
+	resetRegistryForTests()
+	defer resetRegistryForTests()
+
+	plain := &fakeProvider{id: "plain"}
+	withAlbum := &fakeAlbumProvider{fakeProvider: fakeProvider{id: "alb"}}
+	Register(plain)
+	Register(withAlbum)
+
+	if got := len(AllProviders()); got != 2 {
+		t.Fatalf("AllProviders len = %d, want 2", got)
+	}
+
+	// Type-assert to count AlbumCoverProvider implementers — mimics
+	// what SettingsService.EnabledAlbumProviders does internally.
+	var albumProviders []AlbumCoverProvider
+	for _, p := range AllProviders() {
+		if ap, ok := p.(AlbumCoverProvider); ok {
+			albumProviders = append(albumProviders, ap)
+		}
+	}
+	if len(albumProviders) != 1 {
+		t.Fatalf("AlbumCoverProvider count = %d, want 1", len(albumProviders))
+	}
+	if albumProviders[0].ID() != "alb" {
+		t.Errorf("AlbumCoverProvider[0].ID = %q, want %q", albumProviders[0].ID(), "alb")
+	}
+}