Add headroom subscription quota and snapshot polling

[akhilsinghcodes]

Summary

• Add periodic polling of headroom proxy subscription window and quota to track rate limits and usage trends over time
• Persist snapshots to database so metrics survive headroom proxy restarts and history resets
• Expose quota/snapshots via new dashboard API endpoints for UI consumption

Changes

• Backend: New headroom-snapshot-poller.ts that polls /subscription-window, /quota, and /stats every 5 minutes
• Database: New headroom_snapshots table storing subscription window, quota, and stats as JSON blobs with timestamp
• API: Two new endpoints:
  • GET /api/dashboard/headroom/quota — live quota/subscription window from proxy
  • GET /api/dashboard/headroom/snapshots — durable poll history (up to 5000 rows)
• Frontend: New types and API client methods (getHeadroomQuota, getHeadroomSnapshots)
• Startup: Poller initializes on app start (startHeadroomSnapshotPoller())