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>

plugins/testdata/test-metadata-agent/main.go

@@ -134,6 +134,7 @@ func (t *testMetadataAgent) GetSimilarSongsByTrack(input metadata.SimilarSongsBy
 			ID:         "similar-track-id-" + strconv.Itoa(i+1),
 			Name:       "Similar to " + input.Name + " #" + strconv.Itoa(i+1),
 			MBID:       "similar-mbid-" + strconv.Itoa(i+1),
+			ISRC:       "similar-isrc-" + strconv.Itoa(i+1),
 			Artist:     input.Artist,
 			ArtistMBID: "artist-mbid-" + strconv.Itoa(i+1),
 		})