feat(sub): per-client external links and remote subscriptions

Add a Links tab to the client form for attaching third-party share
links and remote subscription URLs per client. They are merged into
the client's raw/JSON/Clash subscription output: links are emitted
verbatim and parsed for JSON/Clash; subscription URLs are fetched
(cached, with a short timeout) and their configs merged in.

i18n keys added across all 13 locales.

internal/web/translation/es-ES.json

@@ -647,6 +647,12 @@
     "clients": {
       "tabBasics": "Básico",
       "tabCredentials": "Credenciales",
+      "tabLinks": "Enlaces",
+      "linksHint": "Añade enlaces de terceros y URLs de suscripción remotas para incluirlos en la suscripción de este cliente.",
+      "addExternalLink": "Añadir enlace externo",
+      "addExternalSubscription": "Añadir suscripción externa",
+      "noExternalLinks": "Aún no hay enlaces externos.",
+      "noExternalSubscriptions": "Aún no hay suscripciones externas.",
       "add": "Añadir cliente",
       "edit": "Editar cliente",
       "submitAdd": "Añadir cliente",