fix(participants): preserve later non-empty CreditedAs on dedup merge

Participants.add deduplicates by ID+SubRole. The same artist appearing
on multiple tracks of one album would silently keep only the
first-seen CreditedAs, so the album-level participants JSON could
end up with a credit from one track applied to all tracks of that
album.

Change the dedup to update the existing entry's CreditedAs when a
later occurrence has a non-empty value. Per-track media_file
participants are unaffected (each track's JSON is built
independently). The album-level merge result is inherently lossy
when tracks differ, but no longer silently drops a meaningful credit
based on arrival order.

model/participants.go

@@ -156,16 +156,25 @@ func (p Participants) Merge(other Participants) {
 }
 
 func (p Participants) add(role Role, participants ...Participant) {
-	seen := make(map[string]struct{}, len(p[role]))
-	for _, artist := range p[role] {
-		seen[artist.ID+artist.SubRole] = struct{}{}
+	seen := make(map[string]int, len(p[role]))
+	for i, artist := range p[role] {
+		seen[artist.ID+artist.SubRole] = i
 	}
 	for _, participant := range participants {
 		key := participant.ID + participant.SubRole
-		if _, ok := seen[key]; !ok {
-			seen[key] = struct{}{}
-			p[role] = append(p[role], participant)
+		if idx, ok := seen[key]; ok {
+			// Same artist/sub-role seen before. The merge (e.g. building
+			// album.Participants from per-track participants) is inherently
+			// lossy when tracks differ on CreditedAs, but silently dropping
+			// the later value can mean the wrong credit ends up on the album.
+			// Prefer the most recently observed non-empty CreditedAs.
+			if participant.CreditedAs != "" {
+				p[role][idx].CreditedAs = participant.CreditedAs
+			}
+			continue
 		}
+		seen[key] = len(p[role])
+		p[role] = append(p[role], participant)
 	}
 }
 

model/participants_test.go

@@ -116,6 +116,40 @@ var _ = Describe("Participants", func() {
 				RoleAlbumArtist: []Participant{_p("3", "AlbumArtist1"), _p("4", "AlbumArtist2"), _p("7", "AlbumArtist3"), _p("8", "AlbumArtist4")},
 			}))
 		})
+
+		It("upgrades CreditedAs from a later occurrence when the dedup key matches", func() {
+			// Same artist (same ID+SubRole) credited differently on two tracks.
+			// The merge is inherently lossy; the later non-empty credit wins so
+			// it isn't silently dropped just because of arrival order.
+			p1 := Participants{
+				RoleArtist: []Participant{
+					{Artist: Artist{ID: "a1", Name: "Aphex Twin"}, CreditedAs: "AFX"},
+				},
+			}
+			p2 := Participants{
+				RoleArtist: []Participant{
+					{Artist: Artist{ID: "a1", Name: "Aphex Twin"}, CreditedAs: "Aphex Twin"},
+				},
+			}
+			p1.Merge(p2)
+			Expect(p1[RoleArtist]).To(HaveLen(1))
+			Expect(p1[RoleArtist][0].CreditedAs).To(Equal("Aphex Twin"))
+		})
+
+		It("does not overwrite an existing CreditedAs with an empty one", func() {
+			p1 := Participants{
+				RoleArtist: []Participant{
+					{Artist: Artist{ID: "a1", Name: "Aphex Twin"}, CreditedAs: "AFX"},
+				},
+			}
+			p2 := Participants{
+				RoleArtist: []Participant{
+					{Artist: Artist{ID: "a1", Name: "Aphex Twin"}}, // empty CreditedAs
+				},
+			}
+			p1.Merge(p2)
+			Expect(p1[RoleArtist][0].CreditedAs).To(Equal("AFX"))
+		})
 	})
 
 	Describe("Hash", func() {