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/auth.graphql

@@ -6,3 +6,13 @@ input LoginInput {
 type AuthPayload {
   user: User!
 }
+
+extend type Query {
+  me: User
+}
+
+extend type Mutation {
+  login(input: LoginInput!): AuthPayload!
+  logout: Boolean!
+  refreshToken: AuthPayload!
+}