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

utils/response.go

@@ -41,3 +41,12 @@ func Conflict(c *fiber.Ctx, msg string) error {
 func Internal(c *fiber.Ctx, msg string) error {
 	return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"success": false, "message": msg})
 }
+
+// Fail is the error response for clients that branch on a stable, machine-
+// readable code rather than the human message. The message stays for logs and
+// fallback display; `code` is the contract (see constants.Err*). Use this on
+// endpoints whose failures the app has to distinguish programmatically; the
+// bare BadRequest/NotFound/... helpers remain fine where it doesn't.
+func Fail(c *fiber.Ctx, status int, code, msg string) error {
+	return c.Status(status).JSON(fiber.Map{"success": false, "code": code, "message": msg})
+}