feat: add ISRC matching for similar songs (#4946)

* feat: add ISRC support to similar songs matching and plugin interface

Add ISRC (International Standard Recording Code) as a high-priority
identifier in the provider matching algorithm, alongside MBID. The
matching pipeline now uses four strategies in priority order:
ID > MBID > ISRC > Title+Artist fuzzy match.

- Add ISRC field to agents.Song struct
- Add ISRC field to plugin capability SongRef (Go, Rust PDKs)
- Add loadTracksByISRC using json_tree query on tags column
- Integrate ISRC into matchSongsToLibrary, selectBestMatchingSongs,
  and buildTitleQueries

https://claude.ai/code/session_01Dd4mTq1VQZag4RNjCVusiF

* chore: regenerate plugin schema after ISRC addition

Run `make gen` to update the generated YAML schema for the
metadata agent capability with the new ISRC field on SongRef.

https://claude.ai/code/session_01Dd4mTq1VQZag4RNjCVusiF

* feat(mediafile): add GetAllByTags method to MediaFileRepository for tag-based retrieval

Signed-off-by: Deluan <deluan@navidrome.org>

* feat(provider): speed up track matching by incorporating prior matches in ISRC and MBID lookups

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>
Co-authored-by: Claude <noreply@anthropic.com>

core/external/extdata_helper_test.go

@@ -92,6 +92,11 @@ func (m *mockMediaFileRepo) Get(id string) (*model.MediaFile, error) {
 	return args.Get(0).(*model.MediaFile), args.Error(1)
 }
 
+// GetAllByTags implements model.MediaFileRepository.
+func (m *mockMediaFileRepo) GetAllByTags(_ model.TagName, _ []string, options ...model.QueryOptions) (model.MediaFiles, error) {
+	return m.GetAll(options...)
+}
+
 // GetAll implements model.MediaFileRepository.
 func (m *mockMediaFileRepo) GetAll(options ...model.QueryOptions) (model.MediaFiles, error) {
 	argsSlice := make([]interface{}, len(options))