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/job_app_reference.graphql

@@ -21,3 +21,13 @@ input UpdateJobAppReferenceInput {
   value: String
   sortOrder: Int
 }
+
+extend type Query {
+  jobAppReferences: [JobAppReference!]!
+}
+
+extend type Mutation {
+  createJobAppReference(input: CreateJobAppReferenceInput!): JobAppReference!
+  updateJobAppReference(id: ID!, input: UpdateJobAppReferenceInput!): JobAppReference!
+  deleteJobAppReference(id: ID!): Boolean!
+}