feat: miler self-set PIN on first login; no console-set default PIN

Milers now choose their own PIN the first time they log in, instead of the
console assigning a shared default:

- CreateMiler always creates a rider with an empty Password (PIN field removed
  from MilerCreateRequest); any client-supplied PIN is ignored, making
  "riders set their own PIN" a backend invariant, not a console convention.
- LoginMiler returns `pin_set` so the app routes to enter-PIN vs set-PIN.
- New POST /miler/set-pin (SetMilerPin): self-service first PIN, allowed ONLY
  when the account has none yet (409 otherwise, so it can't overwrite/take over
  an active account), then logs the rider in. Self-service and throttle-only is
  safe because of that guard; OTP-gate it once the SMS gateway is live.
- verify-pin and set-pin share issueMilerSession so the two success responses
  can't drift.

Also switches BookingPickupComplete's timestamp to DBNow() (IST) so the
compatibility-flow inwardedat matches the reconciliation windows.

Existing riders keep their PIN and are unaffected; blanking their password to
move them onto self-set is a separate, deliberate DB step.

go build, go vet and go test ./... all pass.

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

routes/routes.go

@@ -175,6 +175,11 @@ func RegisterRoutes(app *fiber.App, cfg *config.Config) {
 	miler := api.Group("/miler")
 	miler.Post("/login", authThrottle, controllers.LoginMiler(cfg))
 	miler.Post("/verify-pin", authThrottle, controllers.VerifyMilerPin(cfg))
+	// First-login PIN creation is self-service (unlike reset-pin below), and safe
+	// to leave unauthenticated because SetMilerPin refuses to overwrite an
+	// existing PIN — it only works on an account that has none yet. authThrottle
+	// still caps abuse of the phone-number probe.
+	miler.Post("/set-pin", authThrottle, controllers.SetMilerPin(cfg))
 	// PIN reset is console-operated, NOT self-service: ResetMilerPin overwrites
 	// the PIN given only a phone number, and phone numbers are the miler login
 	// identifier rather than a secret. Left unauthenticated, two calls