Fix IONOS DNS provider field name (api_token -> auth_api_token)

The IONOS libdns provider uses auth_api_token as the JSON field name,
not api_token. This caused Caddy to reject the config with
"unknown field api_token".

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/lib/dns-providers.ts

@@ -177,7 +177,7 @@ export const DNS_PROVIDERS: DnsProviderDefinition[] = [
     docsUrl: "https://github.com/caddy-dns/ionos",
     modulePath: "github.com/caddy-dns/ionos",
     fields: [
-      { key: "api_token", label: "API Token", type: "password", required: true, placeholder: "prefix.secret" },
+      { key: "auth_api_token", label: "API Token", type: "password", required: true, placeholder: "prefix.secret" },
     ],
   },
   {