feat(panel-go): monthly traffic reset for customers

- trafficResetDay/Period/At on the shared customer record (omitempty,
  inert for the Node panel, which preserves unknown fields)
- housekeeping applies due resets before quota evaluation: period-key
  idempotency, short-month clamping (31st fires Feb 28), missed-period
  catch-up after panel downtime, traffic suspensions auto-resume
- first sight of a schedule only arms the period marker, never zeroes;
  API create/PATCH accept trafficResetDay (0=off) and re-arm on change
- UI: reset-day input + last-reset display in the customer form
- tests: calendar math, apply pass, API lifecycle, persistence;
  full Go/Node suites green, difftest formal 50/50

panel-go/handlers.go

@@ -1652,6 +1652,10 @@ func (a *App) handleAdminApi(w http.ResponseWriter, r *http.Request, pathname st
 			if err != nil {
 				return nil, err
 			}
+			resetDay, err := ParseResetDay(body["trafficResetDay"])
+			if err != nil {
+				return nil, err
+			}
 			next := &Customer{
 				ID:                NewID("cus"),
 				SubscriptionToken: RandomToken(32),
@@ -1665,11 +1669,15 @@ func (a *App) handleAdminApi(w http.ResponseWriter, r *http.Request, pathname st
 				ExpiresAt:         expires,
 				IPLimit:           int64(ipLimit),
 				DeviceLimit:       int64(deviceLimit),
+				TrafficResetDay:   resetDay,
 				Tags:              sliceStrings(asIds(body["tags"]), 20),
 				Notes:             CleanText(body["notes"], 1000),
 				CreatedAt:         NowISO(),
 				UpdatedAt:         NowISO(),
 			}
+			// Arm the monthly-reset schedule: it starts with the next
+			// reset instant, it never fires retroactively at creation.
+			ArmTrafficReset(next, time.Now())
 			d.Customers = append(d.Customers, next)
 			Audit(d, actor, "create_customer", next.ID, map[string]any{"name": next.Name})
 			return cloneJSONValue(next), nil
@@ -1736,6 +1744,20 @@ func (a *App) handleAdminApi(w http.ResponseWriter, r *http.Request, pathname st
 			if _, ok := body["tags"]; ok {
 				item.Tags = sliceStrings(asIds(body["tags"]), 20)
 			}
+			if _, ok := body["trafficResetDay"]; ok {
+				v, err := ParseResetDay(body["trafficResetDay"])
+				if err != nil {
+					return nil, err
+				}
+				if v != item.TrafficResetDay {
+					item.TrafficResetDay = v
+					// A changed schedule is re-armed rather than fired:
+					// it takes effect from the next reset instant. An
+					// unchanged value (the UI echoes it on every edit)
+					// leaves the marker — and any pending reset — alone.
+					ArmTrafficReset(item, time.Now())
+				}
+			}
 			reason := CustomerBlockReason(d, item)
 			statusWanted := JSString(body["status"])
 			if statusWanted == "active" && reason != "" {