fix: strip null bytes from ID3v2 TXXX/UFID tag values

The dhowden/tag library's Comm.Text and UFID.Identifier fields
include trailing null bytes from the C-style strings in the ID3v2
binary format. strings.TrimSpace doesn't strip \x00, so MBIDs
stored from MP3 files had invisible null bytes appended.

This caused WHERE mbid = ? queries to fail — the stored value
'uuid\x00' didn't match the clean 'uuid' from search results.
FLAC files (Vorbis comments with plain strings) were unaffected.

Fix: use strings.TrimRight with explicit \x00 in the cutset.
Requires a full rescan to fix existing corrupted MBIDs.

backend/metadata/tags.go

@@ -148,13 +148,14 @@ func extractMBIDs(raw map[string]interface{}, meta *TrackMetadata) {
 		case *tag.Comm:
 			// ID3v2 TXXX frames — Description is the tag name.
 			if val != nil && val.Description != "" {
-				normalized[strings.ToLower(val.Description)] = strings.TrimSpace(val.Text)
+				text := strings.TrimRight(val.Text, "\x00 \t\n\r")
+				normalized[strings.ToLower(val.Description)] = text
 			}
 
 		case *tag.UFID:
 			// ID3v2 UFID frame — MusicBrainz recording ID.
 			if val != nil && val.Provider == "http://musicbrainz.org" {
-				meta.RecordingMBID = strings.TrimSpace(string(val.Identifier))
+				meta.RecordingMBID = strings.TrimRight(string(val.Identifier), "\x00 \t\n\r")
 			}
 		}
 	}