Implement P5.19.a §13.19 Admin UI Overview section enhancements:
- Add "Recent Canary Failures" card to Overview section
- Displays up to 5 most recent failed canaries
- Shows canary name, index, failed assertion count, and time of failure
- Shows success message when all canaries are passing
- Add "CDC Backlog" card to Overview section
- Displays pending CDC event count
- Shows warning when backlog exists
- Add fetchCanaryStatus() and fetchCDCStatus() API functions
- Add renderCanaryFailures() and renderCDCBacklog() rendering functions
- Add formatTimeAgo() helper function for relative time display
- Update refreshData() to fetch canary/CDC status on Overview section
Data sourced from GET /_miroir/canaries endpoint (per plan §13.18).
Closes: miroir-uhj.19.1
5095faa613