Split schema.graphql and schema.resolvers.go into per-domain files

Move Query/Mutation field declarations from the monolithic schema.graphql
into each domain's .graphql file using extend type, so gqlgen places
resolvers in the matching *.resolvers.go files. Extract helper functions
into *_helpers.go files to prevent gqlgen from commenting them out.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/graph/spotify_helpers.go

@@ -0,0 +1,51 @@
+package graph
+
+import (
+	"adam-french.co.uk/backend/graph/model"
+	"github.com/zmb3/spotify/v2"
+)
+
+func mapSpotifyImages(images []spotify.Image) []*model.SpotifyImage {
+	result := make([]*model.SpotifyImage, len(images))
+	for i, img := range images {
+		result[i] = &model.SpotifyImage{URL: img.URL}
+	}
+	return result
+}
+
+func mapSpotifyTrack(track *spotify.FullTrack) *model.SpotifyTrack {
+	artists := make([]*model.SpotifyArtist, len(track.Artists))
+	for i, a := range track.Artists {
+		artists[i] = &model.SpotifyArtist{Name: a.Name}
+	}
+	return &model.SpotifyTrack{
+		Name:    track.Name,
+		Artists: artists,
+		Album: &model.SpotifyAlbum{
+			Name:   track.Album.Name,
+			Images: mapSpotifyImages(track.Album.Images),
+		},
+	}
+}
+
+func mapRecentItems(items []spotify.RecentlyPlayedItem) []*model.SpotifyRecentItem {
+	result := make([]*model.SpotifyRecentItem, len(items))
+	for i, item := range items {
+		artists := make([]*model.SpotifyArtist, len(item.Track.Artists))
+		for j, a := range item.Track.Artists {
+			artists[j] = &model.SpotifyArtist{Name: a.Name}
+		}
+		result[i] = &model.SpotifyRecentItem{
+			PlayedAt: item.PlayedAt,
+			Track: &model.SpotifyTrack{
+				Name:    item.Track.Name,
+				Artists: artists,
+				Album: &model.SpotifyAlbum{
+					Name:   item.Track.Album.Name,
+					Images: mapSpotifyImages(item.Track.Album.Images),
+				},
+			},
+		}
+	}
+	return result
+}