feat(requests): add request history tracking with persistence

- Add requestRecord type to track account/model/tokens/credits per request
- Circular buffer (200 capacity) for recent requests
- RecordRequest hooks in all success/failure paths
- Persist to data/observe/observe.json every 5min + on shutdown
- Load on startup to preserve history across restarts
- New /observe/recent-requests API endpoint
- New Requests tab in admin UI with table view
- 12 i18n keys (zh/en) for requests UI