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/message.graphql

@@ -5,3 +5,7 @@ type Message {
   fileUrl: String
   createdAt: Time!
 }
+
+extend type Query {
+  messages: [Message!]!
+}