Add four test functions using httptest.NewServer:
- TestApplyLabel: verifies POST request path, auth header, label IDs
in body, and cache invalidation after success
- TestApplyLabel_Error: verifies 404 error propagation
- TestSubmitReview: verifies POST path, event/body fields, and cache
invalidation after success
- TestSubmitReview_Error: verifies 422 error propagation
Closesleeworks-agents/gitea-mobile#127
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>