feat(loyalty): push a locked PIN through FCM

A customer whose PIN locks after five wrong attempts is now told by a push
through FCM instead of WhatsApp (docs/prd-point-coin.md F11), to every
device registered at /customer/devices. The push is titled "PIN terkunci",
says until when it is locked, and carries type PIN_LOCKED and locked_until
in its data so the app can offer the PIN reset. As before, only the attempt
that reached the limit sends it, and a failure to send is logged without
affecting the lock.

OtpProcessor.SendWhatsAppMessage was only there for this alert and is
removed; OTPs still go out by WhatsApp.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/processor/customer_pin_processor_db_test.go

@@ -68,10 +68,10 @@ type alerterFake struct {
 	messages []string
 }
 
-func (f *alerterFake) SendWhatsAppMessage(_ string, message string) error {
+func (f *alerterFake) Notify(_ context.Context, _ uuid.UUID, _, body string, _ map[string]string) error {
 	f.mu.Lock()
 	defer f.mu.Unlock()
-	f.messages = append(f.messages, message)
+	f.messages = append(f.messages, body)
 	return nil
 }