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/gitea_helpers.go

@@ -0,0 +1,17 @@
+package graph
+
+import (
+	"adam-french.co.uk/backend/graph/model"
+	"adam-french.co.uk/backend/services"
+)
+
+func mapGiteaFeed(feed *services.GiteaFeedResponse) *model.GiteaFeedItem {
+	return &model.GiteaFeedItem{
+		AvatarURL:     feed.ActUser.AvatarURL,
+		RepoURL:       feed.Repo.HTMLURL,
+		RepoName:      feed.Repo.FullName,
+		OpType:        feed.OpType,
+		CommitMessage: services.ParseCommitMessage(feed.Content),
+		CreatedAt:     feed.Created,
+	}
+}