feat: miler delivery-leg fixes — consignmentid, auto route sequencing, admin consignment status

Miler app P0 + contract gaps found in the live audit:

- GET /miler/bookings now returns consignmentid + consignmentstatus on every
  row (nullable), so the app can call deliver/skip/start-delivery straight from
  the list. /miler/assignments is the active-only queue, so this is the
  authoritative fix for stops that have moved onto the delivery leg.
- GET /miler/bookings now returns sequencedat per row: non-null means the
  console/optimizer fixed this stop's order and the app follows step exactly;
  null means no route assigned and the app may fall back to nearest-first.
- Route sequencing (internal/routing) now runs automatically after every
  assignment — customer auto-assign, express auto-assign, manual assign, and
  accept — via SequenceMilerStopsAsync (fire-and-forget, no-op below two active
  stops). Previously only hub batch-assign sequenced, so most riders saw step=0.
- GET /admin/bookings now surfaces the live consignmentstatus alongside the
  frozen booking status, so a Converted_To_Consignment booking can still show
  Out_for_Delivery / Delivered instead of a generic "Active".

Two-step hyperlocal flow (Arrived_At_Pickup, Collected_By_Miler, start-delivery)
stays gated behind MILER_COLLECTED_STATE_ENABLED (default off) until the app
ships; consignmentid/status, GET /miler/consignments/:id, stable error codes and
Idempotency-Key handling are unconditional and safe on the current app.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WRaFH5hMRqmUQvVPQsyjZD

constants/constants.go

@@ -33,6 +33,7 @@ const (
 	BookingCreated              = "Created"
 	BookingMilerAssigned        = "Miler_Assigned"
 	BookingPickupScheduled      = "Pickup_Scheduled"
+	BookingArrivedAtPickup      = "Arrived_At_Pickup" // miler is at the pickup point, parcel not yet collected
 	BookingPickedUp             = "Picked_Up"
 	BookingConvertedConsignment = "Converted_To_Consignment"
 	BookingCancelled            = "Cancelled"
@@ -40,8 +41,15 @@ const (
 
 // Consignment Statuses
 const (
-	ConsignmentCreated          = "Created"
-	ConsignmentInwardedAtHub    = "Inwarded_at_Hub"
+	ConsignmentCreated       = "Created"
+	ConsignmentInwardedAtHub = "Inwarded_at_Hub"
+	// ConsignmentCollectedByMiler is the intermediate state for a hyperlocal
+	// parcel: the miler has collected it but has NOT yet started the final-mile
+	// run. It sits between pickup and Out_for_Delivery so the console can tell
+	// "collected, waiting to leave" apart from "actively delivering" — before
+	// this existed a hyperlocal pickup jumped straight to Out_for_Delivery and
+	// looked active the instant it was collected. StartDelivery moves it on.
+	ConsignmentCollectedByMiler = "Collected_By_Miler"
 	ConsignmentTripsheetLoaded  = "Tripsheet_Loaded"
 	ConsignmentInTransit        = "In_Transit"
 	ConsignmentOutForDelivery   = "Out_for_Delivery"
@@ -52,6 +60,23 @@ const (
 	ConsignmentDamaged          = "Damaged"
 )
 
+// Machine-readable error codes returned to the miler app in the "code" field of
+// 4xx responses, so the client can branch on a stable identifier instead of
+// parsing human-readable messages. Add here, never inline.
+const (
+	ErrInvalidInput           = "INVALID_INPUT"
+	ErrBookingNotFound        = "BOOKING_NOT_FOUND"
+	ErrBookingNotAssigned     = "BOOKING_NOT_ASSIGNED"
+	ErrConsignmentNotFound    = "CONSIGNMENT_NOT_FOUND"
+	ErrConsignmentNotAssigned = "CONSIGNMENT_NOT_ASSIGNED"
+	ErrInvalidState           = "INVALID_STATE"     // action not allowed from the entity's current status
+	ErrAlreadyPickedUp        = "ALREADY_PICKED_UP" // pre-pickup action attempted after pickup
+	ErrOtpRequired            = "OTP_REQUIRED"
+	ErrOtpInvalid             = "OTP_INVALID"
+	ErrIdempotencyInProgress  = "IDEMPOTENCY_IN_PROGRESS" // an identical keyed request is still running
+	ErrEmailInUse             = "EMAIL_IN_USE"
+)
+
 // Payment Modes
 const (
 	PaymentModeCash   = "Cash"