feat(quota): add tunnel traffic quota with daily/monthly limits (#291) (#308)

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

go-backend/internal/http/handler/jobs.go

@@ -136,6 +136,7 @@ func (h *Handler) runResetAndExpiryJob(now time.Time) {
 	}
 
 	h.resetMonthlyFlow(now)
+	h.resetTunnelQuotaWindows(now)
 	h.disableExpiredUsers(now.UnixMilli())
 	h.disableExpiredUserTunnels(now.UnixMilli())
 }