Adds a new diagnostics page under Config menu to monitor and manage
Oban job queues. Helps troubleshoot stuck or failed jobs without
needing to query the database directly.
Features:
- System overview (pending downloads, total media, sources, db size)
- Queue health status with running/available/scheduled/retryable counts
- Stuck jobs detection (executing > 30 min) with reset/cancel actions
- Failed jobs list with error details and bulk reset option
