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/user.graphql

@@ -10,3 +10,14 @@ input CreateUserInput {
   username: String!
   password: String!
 }
+
+extend type Query {
+  users: [User!]!
+  user(id: ID!): User
+}
+
+extend type Mutation {
+  createUser(input: CreateUserInput!): User!
+  deleteUser(id: ID!): User!
+  setUserAdmin(id: ID!, admin: Boolean!): User!
+}