feat(core): add MusicBrainz orchestration methods to QbzCore

Adds qbz-integrations as a dependency of qbz-core and embeds a
MusicBrainzClient on the QbzCore struct (initialized in new()). The
client's own enabled flag governs whether requests fire; calls return
the integration-layer ServiceUnavailable error when disabled.

New methods on QbzCore<A>:

  musicbrainz_is_enabled()
  musicbrainz_set_enabled(enabled)
  musicbrainz_resolve_artist(name) -> Option<ResolvedArtist>
  musicbrainz_get_artist_metadata(mbid) -> ArtistMetadata
  musicbrainz_get_artist_relationships(mbid) -> ArtistRelationships

The metadata method calls location::extract_metadata on the full artist
response and then resolves the real country from the begin_area
hierarchy when a city-level location was found (MB's "country" field is
where the artist is currently active, not where they were born/formed).
The relationships method splits relations into members / past_members /
groups / collaborators by relation_type and direction, mirroring
v2_musicbrainz_get_artist_relationships exactly.

No cache wiring yet — every call hits the network. A cache wrapper can
be plugged in later via a setter without changing the method
signatures.

cc

src-tauri/Cargo.lock

@@ -5118,6 +5118,7 @@ dependencies = [
  "async-trait",
  "log",
  "qbz-audio",
+ "qbz-integrations",
  "qbz-models",
  "qbz-player",
  "qbz-qobuz",