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/model/models_gen.go

@@ -34,6 +34,15 @@ type CreateUserInput struct {
 	Password string `json:"password"`
 }
 
+type GiteaFeedItem struct {
+	AvatarURL     string    `json:"avatarUrl"`
+	RepoURL       string    `json:"repoUrl"`
+	RepoName      string    `json:"repoName"`
+	OpType        string    `json:"opType"`
+	CommitMessage string    `json:"commitMessage"`
+	CreatedAt     time.Time `json:"createdAt"`
+}
+
 type LoginInput struct {
 	Username string `json:"username"`
 	Password string `json:"password"`