feat: extract MusicBrainz IDs from audio tags and store in library DB

Migration 13 adds nullable mbid TEXT columns to artists,
release_groups, and recordings with partial indexes.

Metadata extraction (tags.go) now reads MusicBrainz IDs from Raw()
tags — handles both Vorbis (musicbrainz_artistid) and ID3v2
(MusicBrainz Artist Id) key formats.

Scan pipeline (library.go) updates MBIDs after entity upsert via
raw SQL UPDATE. Only sets mbid if currently NULL (preserves existing
values on rescan).

LibraryMBIDIndex (librarymbid.go) provides:
- CheckMBIDs: batch lookup for 'In Library' badges
- GetArtistMBID: single artist name→MBID lookup
- AllArtistMBIDs: full dump for search index Tier 3

MBIDs will be populated on next library rescan. Existing files
need a rescan to backfill.

backend/metadata/tags.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"strings"
 
 	"github.com/dhowden/tag"
 )
@@ -30,6 +31,13 @@ type TrackMetadata struct {
 	Lyrics  string
 	Comment string
 
+	// MusicBrainz IDs (from tags, may be empty)
+	ArtistMBID       string
+	AlbumArtistMBID  string
+	ReleaseGroupMBID string
+	ReleaseMBID      string
+	RecordingMBID    string
+
 	// Cover art (if present)
 	Picture *PictureData
 
@@ -90,6 +98,9 @@ func ExtractTagsFromReader(r io.ReadSeeker) (*TrackMetadata, error) {
 		FileFormat:  string(m.FileType()),
 	}
 
+	// Extract MusicBrainz IDs from raw tags.
+	extractMBIDs(m.Raw(), meta)
+
 	// Extract picture if present
 	if pic := m.Picture(); pic != nil {
 		meta.Picture = &PictureData{
@@ -101,3 +112,52 @@ func ExtractTagsFromReader(r io.ReadSeeker) (*TrackMetadata, error) {
 
 	return meta, nil
 }
+
+// mbidTagKeys maps TrackMetadata field names to the possible raw
+// tag keys across formats (ID3v2 TXXX, Vorbis, MP4).  All keys
+// are lowercased for case-insensitive matching.
+var mbidTagKeys = map[string][]string{
+	"ArtistMBID":       {"musicbrainz_artistid", "musicbrainz artist id"},
+	"AlbumArtistMBID":  {"musicbrainz_albumartistid", "musicbrainz album artist id"},
+	"ReleaseGroupMBID": {"musicbrainz_releasegroupid", "musicbrainz release group id"},
+	"ReleaseMBID":      {"musicbrainz_albumid", "musicbrainz album id"},
+	"RecordingMBID":    {"musicbrainz_trackid", "musicbrainz recording id"},
+}
+
+// extractMBIDs populates the MBID fields of meta from the raw tag
+// map.  Handles varying key names across ID3v2, Vorbis, and MP4.
+func extractMBIDs(raw map[string]interface{}, meta *TrackMetadata) {
+	if len(raw) == 0 {
+		return
+	}
+
+	// Build a lowercased key → value map for case-insensitive lookup.
+	normalized := make(map[string]string, len(raw))
+
+	for k, v := range raw {
+		if s, ok := v.(string); ok {
+			normalized[strings.ToLower(k)] = s
+		}
+	}
+
+	for field, keys := range mbidTagKeys {
+		for _, key := range keys {
+			if val, ok := normalized[key]; ok && val != "" {
+				switch field {
+				case "ArtistMBID":
+					meta.ArtistMBID = val
+				case "AlbumArtistMBID":
+					meta.AlbumArtistMBID = val
+				case "ReleaseGroupMBID":
+					meta.ReleaseGroupMBID = val
+				case "ReleaseMBID":
+					meta.ReleaseMBID = val
+				case "RecordingMBID":
+					meta.RecordingMBID = val
+				}
+
+				break
+			}
+		}
+	}
+}