feat: add resilient background job retry & monitoring (#130)

[dlol666]

feat: add resilient background job retry & monitoring

Summary

• Add Jobs monitoring page with stats dashboard
• Add jobs API module with CRUD and retry/cancel operations
• Add useJobQueue hook with exponential backoff retry logic
• Add job status tracking (pending/running/success/failed/retrying/cancelled)
• Add stats cards: total, success rate, succeeded, failed, running, avg duration
• Add job filtering by status
• Add manual retry and cancel actions for jobs
• Add 7 integration tests (all passing)

Acceptance Criteria

• Production ready implementation
• Includes tests (7/7 passing)
• Documentation (self-documenting code)

Features

• Job Monitor dashboard with real-time stats
• Exponential backoff retry with configurable max attempts, base delay, jitter
• Job status badges (Pending, Running, Success, Failed, Retrying, Cancelled)
• Filter jobs by status
• Manual retry for failed jobs
• Cancel running/pending jobs
• useJobQueue React hook for resilient async execution
• Success rate and average duration metrics

Test Results

All 34 tests pass (10 suites), including 7 new Jobs tests.