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/types.go

@@ -532,6 +532,9 @@ type Customer struct {
 	UsedBytes            int64                 `json:"usedBytes"`
 	UsedUplinkBytes      int64                 `json:"usedUplinkBytes"`
 	UsedDownlinkBytes    int64                 `json:"usedDownlinkBytes"`
+	TrafficResetDay      int                   `json:"trafficResetDay,omitempty"`
+	TrafficResetPeriod   string                `json:"trafficResetPeriod,omitempty"`
+	TrafficResetAt       *string               `json:"trafficResetAt,omitempty"`
 	ExpiresAt            *string               `json:"expiresAt"`
 	IPLimit              int64                 `json:"ipLimit"`
 	DeviceLimit          int64                 `json:"deviceLimit"`