Añade gold y transfer: patrón de pago unificado con fulfill + metadata

- lib/stripe.ts: createCheckoutSession acepta metadata; claimPaidCheckout la
  recupera de la sesión Stripe y la devuelve.
- lib/paid-services.ts: config unificado con fulfill(character, meta) + extraFields.
  gold -> UPDATE characters money (BD directa, no SOAP), transfer -> SOAP
  .char changeaccount. Los 5 simples usan soapFulfill.
- checkout [service]: lee extraFields -> metadata, precio depende de metadata (gold),
  valida precio>0. service-success llama cfg.fulfill(name, metadata).
- prices.ts: getTransferPrice, getGoldOptions/goldPriceFor (home_goldprice).
- GoldForm (personaje + cantidad) y TransferForm (personaje + destino); páginas
  /gold y /transfer. Catálogos Paid.gold/transfer.

Verificado: /gold /transfer redirigen a login, checkout 401 sin sesión, home OK.
NOTA: transfer no valida aún el token de seguridad ni reglas AC (nivel>=55, DK).

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

web-next/messages/en.json

@@ -162,6 +162,19 @@
       "pay": "Level up to 80 for {price} €",
       "success": "Character {name} has been leveled to 80."
     },
-    "error": "The operation could not be completed. If you were charged, contact support."
+    "error": "The operation could not be completed. If you were charged, contact support.",
+    "gold": {
+      "title": "Buy gold",
+      "buy": "Buy gold",
+      "selectAmount": "Select the amount",
+      "option": "{amount} gold — {price} €",
+      "success": "Gold has been added to character {name}."
+    },
+    "transfer": {
+      "title": "Transfer character",
+      "pay": "Transfer for {price} €",
+      "destination": "Destination account",
+      "success": "Character {name} has been transferred to the destination account."
+    }
   }
 }

web-next/messages/es.json

@@ -162,6 +162,19 @@
       "pay": "Subir a nivel 80 por {price} €",
       "success": "El personaje {name} ha sido subido al nivel 80."
     },
-    "error": "No se pudo completar la operación. Si se te ha cobrado, contacta con soporte."
+    "error": "No se pudo completar la operación. Si se te ha cobrado, contacta con soporte.",
+    "gold": {
+      "title": "Adquirir oro",
+      "buy": "Comprar oro",
+      "selectAmount": "Selecciona la cantidad",
+      "option": "{amount} oro — {price} €",
+      "success": "El oro ha sido añadido al personaje {name}."
+    },
+    "transfer": {
+      "title": "Transferir personaje",
+      "pay": "Transferir por {price} €",
+      "destination": "Cuenta de destino",
+      "success": "El personaje {name} ha sido transferido a la cuenta de destino."
+    }
   }
 }