Callers now supply their own Gitea PAT as a Bearer token; the server validates
it against GET /api/v1/user and threads it through context to all downstream
Gitea API calls. GITEA_API_TOKEN env var and the GiteaAPIToken config field are
removed.
Shared LRU avoids repeated Gitea calls for default-branch resolution;
the simple stdlib map alternative would race on concurrent access without
a mutex per entry, which is more code than the LRU.
