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/steam_helpers.go

@@ -0,0 +1,22 @@
+package graph
+
+import (
+	"fmt"
+
+	"adam-french.co.uk/backend/graph/model"
+	"adam-french.co.uk/backend/services"
+)
+
+func mapSteamGames(games []services.SteamRecentGame) []*model.SteamGame {
+	result := make([]*model.SteamGame, len(games))
+	for i, g := range games {
+		result[i] = &model.SteamGame{
+			AppID:           g.AppID,
+			Name:            g.Name,
+			Playtime2Weeks:  g.Playtime2Weeks,
+			PlaytimeForever: g.PlaytimeForever,
+			HeaderImageURL:  fmt.Sprintf("https://cdn.akamai.steamstatic.com/steam/apps/%d/header.jpg", g.AppID),
+		}
+	}
+	return result
+}