fix(server/m7-382): MBID extraction never matched dhowden/tag's Raw() keys

The hand-rolled extractor in internal/library/mbids.go looked up keys that
dhowden/tag never produces, for every supported audio format:

  Vorbis (FLAC/OGG): dhowden lowercases keys at parse time (vorbis.go:59),
    we looked up MUSICBRAINZ_ALBUMID. Always missed.
  ID3v2 (MP3): dhowden stores TXXX frames under bare TXXX/TXXX_N keys
    with *tag.Comm values; the Picard tag name is on the value's
    Description field, not part of the key. We looked up
    TXXX:MusicBrainz Album Id. Always missed.
  MP4 (M4A): dhowden stores freeform iTunes atoms under their bare
    sub-name. We looked up ----:com.apple.iTunes:MusicBrainz Album Id.
    Always missed.

Net effect: every album in the m7-380 boot backfill reported
processed=N, healed=0, skipped=N. The m7-379 enricher gate then
short-circuited NULL on every album, so MBCAA was never queried.
Symptom: "all albums skipped" during cover enrichment.

The pre-existing unit tests passed because they hand-built Raw() maps
with the intended-but-incorrect keys, never round-tripping through
dhowden's parser.

Replace the hand-rolled extractor with dhowden's own mbz sub-package,
which knows the per-format Raw() conventions and uses lowercase
canonical keys (mbz.Album, mbz.Artist). Update both call sites to pass
the full tag.Metadata instead of just Raw(). Rewrite the test with a
stubMeta implementing tag.Metadata, asserting against the actual
per-format Raw() shape, plus a regression guard that pins "uppercase
Vorbis keys must not match" so the v0 bug can't sneak back.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/library/mbids.go

@@ -1,59 +1,23 @@
 package library
 
-// MusicBrainz tag keys vary by audio format. dhowden/tag's Raw() map exposes
-// the format-specific entries as-is — so we look up multiple candidates and
-// take the first non-empty match.
+import (
+	"github.com/dhowden/tag"
+	"github.com/dhowden/tag/mbz"
+)
+
+// extractMBIDs reads MusicBrainz album and artist IDs from a tag.Metadata.
 //
-// References:
-//   - ID3v2 (MP3):       TXXX frames keyed by description.
-//   - Vorbis (FLAC/OGG): all-caps comment names.
-//   - MP4 (M4A):         iTunes freeform atoms ("----:com.apple.iTunes:...").
-
-var albumMBIDKeys = []string{
-	"TXXX:MusicBrainz Album Id",
-	"MUSICBRAINZ_ALBUMID",
-	"----:com.apple.iTunes:MusicBrainz Album Id",
-}
-
-var artistMBIDKeys = []string{
-	"TXXX:MusicBrainz Artist Id",
-	"MUSICBRAINZ_ARTISTID",
-	"----:com.apple.iTunes:MusicBrainz Artist Id",
-}
-
-// extractMBIDs reads MusicBrainz album and artist IDs from a tag.Metadata
-// Raw() map. Returns ("", "") when the keys are absent. Multi-value keys
-// (some formats list multiple artist IDs for collaborations) take only the
-// first value — Minstrel uses the primary release artist for MBCAA lookup.
-func extractMBIDs(raw map[string]interface{}) (albumMBID, artistMBID string) {
-	albumMBID = firstStringValue(raw, albumMBIDKeys)
-	artistMBID = firstStringValue(raw, artistMBIDKeys)
-	return
-}
-
-func firstStringValue(raw map[string]interface{}, keys []string) string {
-	for _, k := range keys {
-		v, ok := raw[k]
-		if !ok || v == nil {
-			continue
-		}
-		switch x := v.(type) {
-		case string:
-			if x != "" {
-				return x
-			}
-		case []string:
-			if len(x) > 0 && x[0] != "" {
-				return x[0]
-			}
-		case []interface{}:
-			if len(x) == 0 {
-				continue
-			}
-			if s, ok := x[0].(string); ok && s != "" {
-				return s
-			}
-		}
-	}
-	return ""
+// Delegates to dhowden/tag/mbz, which knows the per-format Raw() conventions:
+//   - Vorbis (FLAC/OGG) keys are lowercased by the parser.
+//   - ID3v2 (MP3) TXXX frames are stored under "TXXX"/"TXXX_N" with
+//     a *tag.Comm value whose Description holds the Picard tag name.
+//   - MP4 (M4A) freeform iTunes atoms are stored under their bare sub-name.
+//
+// We previously hand-rolled this lookup against guessed keys
+// ("MUSICBRAINZ_ALBUMID", "TXXX:MusicBrainz Album Id", ...), none of which
+// dhowden actually produces — so every extraction returned ("",""), which
+// fed an empty MBID into both the scanner and the boot-time backfill.
+func extractMBIDs(m tag.Metadata) (albumMBID, artistMBID string) {
+	info := mbz.Extract(m)
+	return info.Get(mbz.Album), info.Get(mbz.Artist)
 }