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/message.resolvers.go

@@ -21,6 +21,19 @@ func (r *messageResolver) AuthorID(ctx context.Context, obj *models.Message) (in
 	return int(obj.AuthorID), nil
 }
 
+// Messages is the resolver for the messages field.
+func (r *queryResolver) Messages(ctx context.Context) ([]*models.Message, error) {
+	var messages []models.Message
+	if err := r.Store.DB.Order("created_at DESC").Find(&messages).Error; err != nil {
+		return nil, err
+	}
+	result := make([]*models.Message, len(messages))
+	for i := range messages {
+		result[i] = &messages[i]
+	}
+	return result, nil
+}
+
 // Message returns MessageResolver implementation.
 func (r *Resolver) Message() MessageResolver { return &messageResolver{r} }