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/schema/favorite.graphql

@@ -12,3 +12,11 @@ input CreateFavoriteInput {
   name: String!
   link: String
 }
+
+extend type Query {
+  favorites: [Favorite!]!
+}
+
+extend type Mutation {
+  createFavorite(input: CreateFavoriteInput!): Favorite!
+}