refactor(integrations): move MB location and genre helpers to qbz-integrations

Pure-logic move of two helper modules out of src-tauri/src/musicbrainz/
into the qbz-integrations crate, where the MusicBrainzClient and DTOs
already live:

  src-tauri/src/musicbrainz/location_discovery.rs
    -> crates/qbz-integrations/src/musicbrainz/location.rs
  src-tauri/src/musicbrainz/genre_normalization.rs
    -> crates/qbz-integrations/src/musicbrainz/genre.rs

The functions exposed are unchanged: extract_metadata,
compute_affinity_score, build_scene_cache_key, format_life_span_date
(from location); normalize_genre, is_broad_genre, extract_affinity_seeds,
genre_summary (from genre). Imports in src-tauri/src/commands_v2/
integrations.rs are repointed to the new home; the legacy file
declarations are removed from src-tauri/src/musicbrainz/mod.rs. No
behavior change.

Unblocks the Slint side from reaching the same helpers via qbz-core
without depending on src-tauri.

cc

src-tauri/Cargo.lock

@@ -5121,6 +5121,7 @@ dependencies = [
  "qbz-models",
  "qbz-player",
  "qbz-qobuz",
+ "serde",
  "serde_json",
  "thiserror 2.0.18",
  "tokio",