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.resolvers.go

@@ -1,58 +1,18 @@
 package graph
 
+// This file will be automatically regenerated based on the schema, any resolver
+// implementations
+// will be copied through when generating and any unknown code will be moved to the end.
+// Code generated by github.com/99designs/gqlgen version v0.17.88
+
 import (
 	"context"
 	"time"
 
 	"adam-french.co.uk/backend/graph/model"
-	"github.com/zmb3/spotify/v2"
+	spotify "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
-}
-
 // SpotifyListening is the resolver for the spotifyListening field.
 func (r *queryResolver) SpotifyListening(ctx context.Context) (*model.SpotifyPlaying, error) {
 	if r.Store.SpotifyClient == nil {