Move Gitea feed from frontend to backend with cached GraphQL proxy

Replaces direct browser-to-Gitea API calls with a backend service that
proxies and caches the feed (1-min TTL), served via the existing GraphQL
HomeData query. Commit message parsing now happens server-side.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/graph/schema/gitea.graphql

@@ -0,0 +1,8 @@
+type GiteaFeedItem {
+  avatarUrl: String!
+  repoUrl: String!
+  repoName: String!
+  opType: String!
+  commitMessage: String!
+  createdAt: Time!
+}