fix(metadata): preserve source release fields across providers

go_backend/extension_fallback.go

@@ -428,7 +428,9 @@ func DownloadWithExtensionFallback(req DownloadRequest) (*DownloadResponse, erro
 					Name:        req.TrackName,
 					Artists:     req.ArtistName,
 					AlbumName:   req.AlbumName,
+					AlbumArtist: req.AlbumArtist,
 					DurationMS:  req.DurationMS,
+					CoverURL:    req.CoverURL,
 					ISRC:        req.ISRC,
 					ReleaseDate: req.ReleaseDate,
 					TrackNumber: req.TrackNumber,
@@ -436,7 +438,14 @@ func DownloadWithExtensionFallback(req DownloadRequest) (*DownloadResponse, erro
 					DiscNumber:  req.DiscNumber,
 					TotalDiscs:  req.TotalDiscs,
 					ProviderID:  req.Source,
+					AlbumType:   req.AlbumType,
+					Explicit:    req.Explicit,
+					UPC:         req.UPC,
+					Label:       req.Label,
+					Copyright:   req.Copyright,
+					Genre:       req.Genre,
 					Composer:    req.Composer,
+					Comment:     req.Comment,
 				}
 
 				enrichedTrack, err := provider.EnrichTrackForItemID(trackMeta, req.ItemID)
@@ -480,6 +489,7 @@ func DownloadWithExtensionFallback(req DownloadRequest) (*DownloadResponse, erro
 					overlayInt(&req.DiscNumber, enrichedTrack.DiscNumber, "DiscNumber")
 					overlayInt(&req.TotalDiscs, enrichedTrack.TotalDiscs, "TotalDiscs")
 					overlayStr(&req.Composer, enrichedTrack.Composer, "Composer")
+					overlayExtensionReleaseMetadata(&req, *enrichedTrack)
 				}
 			}
 		}
@@ -516,6 +526,7 @@ func DownloadWithExtensionFallback(req DownloadRequest) (*DownloadResponse, erro
 				overlayStr(&req.Genre, track.Genre, "")
 				overlayStr(&req.Label, track.Label, "")
 				overlayStr(&req.Copyright, track.Copyright, "")
+				overlayExtensionReleaseMetadata(&req, track)
 			} else if searchErr != nil {
 				GoLog("[DownloadWithExtensionFallback] Metadata provider search failed (non-fatal): %v\n", searchErr)
 			}

go_backend/extension_fallback_helpers.go

@@ -225,6 +225,22 @@ func overlayStr(dst *string, src, field string) {
 	}
 }
 
+// overlayExtensionReleaseMetadata carries release-level metadata discovered by
+// the source catalog into a provider-agnostic download request. Existing
+// source values always win; enrichment only fills fields that were missing.
+func overlayExtensionReleaseMetadata(req *DownloadRequest, track ExtTrackMetadata) {
+	if req == nil {
+		return
+	}
+	overlayStr(&req.AlbumType, track.AlbumType, "AlbumType")
+	overlayStr(&req.UPC, track.UPC, "UPC")
+	overlayStr(&req.Comment, track.Comment, "Comment")
+	if !req.Explicit && track.Explicit {
+		req.Explicit = true
+		GoLog("[DownloadWithExtensionFallback] Explicit flag from enrichment\n")
+	}
+}
+
 // overlayStrTrim is overlayStr but treats a whitespace-only dst as empty too.
 func overlayStrTrim(dst *string, src string) {
 	if src == "" || strings.TrimSpace(*dst) != "" {

go_backend/extension_fallback_metadata_test.go

@@ -0,0 +1,57 @@
+package gobackend
+
+import "testing"
+
+func TestOverlayExtensionReleaseMetadataFillsMissingRequestFields(t *testing.T) {
+	req := DownloadRequest{AlbumType: "single"}
+	track := ExtTrackMetadata{
+		AlbumType: "album",
+		Explicit:  true,
+		UPC:       "4006381333931",
+		Comment:   "https://music.apple.com/jp/album/1532211596",
+	}
+
+	overlayExtensionReleaseMetadata(&req, track)
+
+	if req.AlbumType != "single" {
+		t.Fatalf("existing album type was overwritten: %q", req.AlbumType)
+	}
+	if !req.Explicit {
+		t.Fatal("explicit flag from source enrichment was not retained")
+	}
+	if req.UPC != track.UPC {
+		t.Fatalf("UPC = %q, want %q", req.UPC, track.UPC)
+	}
+	if req.Comment != track.Comment {
+		t.Fatalf("comment = %q, want %q", req.Comment, track.Comment)
+	}
+
+	response := buildDownloadSuccessResponse(
+		req,
+		DownloadResult{},
+		"amazon",
+		"downloaded",
+		"song.m4a",
+		false,
+	)
+	if response.UPC != track.UPC || response.AlbumType != "single" ||
+		!response.Explicit || response.Comment != track.Comment {
+		t.Fatalf("enriched metadata was lost in download response: %#v", response)
+	}
+}
+
+func TestOverlayExtensionReleaseMetadataDoesNotEraseExistingValues(t *testing.T) {
+	req := DownloadRequest{
+		AlbumType: "ep",
+		Explicit:  true,
+		UPC:       "existing-upc",
+		Comment:   "existing-comment",
+	}
+
+	overlayExtensionReleaseMetadata(&req, ExtTrackMetadata{})
+
+	if req.AlbumType != "ep" || !req.Explicit ||
+		req.UPC != "existing-upc" || req.Comment != "existing-comment" {
+		t.Fatalf("existing release metadata changed: %#v", req)
+	}
+}

go_backend/extension_provider_wrapper.go

@@ -259,6 +259,7 @@ func extensionTrackInput(track *ExtTrackMetadata) map[string]any {
 		"copyright":      track.Copyright,
 		"genre":          track.Genre,
 		"composer":       track.Composer,
+		"comment":        track.Comment,
 		"audio_quality":  track.AudioQuality,
 		"audio_modes":    track.AudioModes,
 		"upc":            track.UPC,