send-gift: permitir pagar con PV (puntos de voto), más caro que PD

Solo en /send-gift se añade una 4ª forma de pago: PV (vote points, gratis por
votar). Cuesta más que PD para preservar su valor: coste_PV = coste_PD ×
VP_PRICE_FACTOR (=2, configurable en lib/pay-with-dpoints).

- lib/dpoints: getVPointsBalance, creditVPoints y spendVPoints (débito atómico
  del campo vp, con FOR UPDATE), espejo de las de PD.
- lib/pay-with-dpoints: VP_PRICE_FACTOR, vpointsCost() y payServiceWithVPoints()
  (descuenta PV, ejecuta el envío y reembolsa si falla).
- gift/checkout: rama provider='vp' (además de 'pd').
- service-success: la entrega inmediata cubre 'pd' y 'vp'.
- PaymentMethodSelect: opción VP opcional (solo si el form pasa vpBalance);
  muestra coste en PV, insuficiencia y el saldo VP.
- SendGiftForm + página: pasan el saldo VP.
- i18n Pay: vp, vpCost, insufficientVp, balanceVp, errors.insufficientVp (es/en).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web-next/messages/en.json

@@ -1920,7 +1920,12 @@
       "fulfillFailed": "The service could not be completed. Your PD balance has been refunded.",
       "notConfigured": "This payment method is not available right now.",
       "invalidRequest": "Invalid request.",
-      "genericError": "An error occurred. Please try again."
-    }
+      "genericError": "An error occurred. Please try again.",
+      "insufficientVp": "You don't have enough vote points for this service."
+    },
+    "vp": "Vote points",
+    "vpCost": "{cost} VP",
+    "insufficientVp": "Insufficient balance ({cost} VP)",
+    "balanceVp": "VP: {balance}"
   }
 }