feat(background-jobs): resilient job retry & monitoring with exponential backoff

[bkennedyshit]

Closes #130

Summary

Adds a resilient background job system with retry logic, monitoring dashboard, and exponential backoff.

Features

• ✅ In-memory job queue with max 3 retries + exponential backoff
• ✅ Job types: export_data, generate_report, sync_transactions
• ✅ Backend endpoints: POST /api/jobs, GET /api/jobs, GET /api/jobs/:id
• ✅ Failure logging with timestamp and error message
• ✅ Frontend JobMonitor component — polls every 10s, shows pending/running/failed/complete
• ✅ Retry count + next retry time displayed for failed jobs
• ✅ Collapsible panel added to Dashboard

Files Changed

• packages/backend/app/routes/jobs.py (new)
• app/src/pages/JobMonitor.tsx (new component)
• app/src/App.tsx (route)
• app/src/components/layout/Navbar.tsx (nav link)