Record the last 10 /v1/ API requests (including errors) in an in-memory
ring buffer. Each entry captures request headers (Authorization masked),
request body, response status, and response body — all up to 1 MB.
- recorder.go: RecentRecorder ring buffer, recordingResponseWriter,
withRecording middleware, getRecent handler
- server.go: add recorder to Server, wrap /v1/ routes, add /api/recent
- admin.html: new 最近请求 tab with lazy-load and expandable cards
- recorder_test.go: 5 tests (ring buffer, ordering, capture, errors, API)
