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_application.graphql

@@ -30,3 +30,14 @@ input UpdateJobApplicationInput {
   notes: String
   appliedAt: Time
 }
+
+extend type Query {
+  jobApplications: [JobApplication!]!
+  jobApplication(id: ID!): JobApplication
+}
+
+extend type Mutation {
+  createJobApplication(input: CreateJobApplicationInput!): JobApplication!
+  updateJobApplication(id: ID!, input: UpdateJobApplicationInput!): JobApplication!
+  deleteJobApplication(id: ID!): Boolean!
+}