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/post.graphql

@@ -16,3 +16,14 @@ input UpdatePostInput {
   title: String!
   content: String!
 }
+
+extend type Query {
+  posts: [Post!]!
+  post(id: ID!): Post
+}
+
+extend type Mutation {
+  createPost(input: CreatePostInput!): Post!
+  updatePost(id: ID!, input: UpdatePostInput!): Post!
+  deletePost(id: ID!): Post!
+}