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/bookmark.graphql

@@ -12,3 +12,12 @@ input CreateBookmarkInput {
   name: String!
   link: String!
 }
+
+extend type Query {
+  bookmarks: [Bookmark!]!
+}
+
+extend type Mutation {
+  createBookmark(input: CreateBookmarkInput!): Bookmark!
+  deleteBookmark(id: ID!): Bookmark!
+}