Implement per-tunnel traffic quota feature:
- Add TunnelQuota model with daily/monthly usage tracking
- Integrate quota enforcement into flow accumulation path
- Pause forwards and disable tunnel when quota exceeded
- Block new forward creation/resume when tunnel quota disabled
- Auto-reset daily/monthly windows at 00:05 via maintenance job
- Add manual reset API endpoint for admins
- Include quota config in tunnel backup/restore
- Add frontend UI for quota settings and usage display
Entire-Checkpoint: e629b27ca437
sagit