v0.168.0: customer-configurable backup window (Mentési időablak)

ONE setting (window start W) drives every nightly leg at fixed, never-stored
offsets: DB dump at W, tier-2 at W+60m, off-box at W+105m (wrap-safe). Precedence
settings > controller.yaml db_dump_schedule > 02:30.

- scheduler.UpdateDaily: retime a daily job at runtime (no restart) via a per-job
  buffered resched chan + a select case in runDailyJob.
- new pure package internal/backupwindow (LegTimes/GateWindow/EffectiveWindow).
- quiesce disk-tier window gate: scheduled cycles run only inside [W+2h,W+6h) with a
  safety valve (age>cadence+24h runs regardless); manual TriggerNow never gated.
  Backend.Due now also returns the backup age (from the agent's own /backup/due).
- backup page: Mentési időablak card (time input + derived leg/gate rows); POST
  /backups/window validates -> saves -> UpdateDaily x3 -> flash.

Tests: 5 groups, all red-proofed. Agent/cadence//backup/due untouched.

controller/internal/quiesce/quiesce_8b2_test.go

@@ -34,8 +34,8 @@ type eventBackend struct {
 	i      int
 }
 
-func (b *eventBackend) Due(context.Context) (bool, error)              { return true, nil }
-func (b *eventBackend) StartBackup(context.Context) (string, error)    { return "job-1", nil }
+func (b *eventBackend) Due(context.Context) (bool, *int64, error)   { return true, nil, nil }
+func (b *eventBackend) StartBackup(context.Context) (string, error) { return "job-1", nil }
 func (b *eventBackend) BackupStatus(context.Context) (string, error) {
 	ph := b.phases[len(b.phases)-1]
 	if b.i < len(b.phases) {