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/database/db.go

@@ -69,6 +69,7 @@ func initModels() error {
 		&model.ApiToken{},
 		&model.ClientRecord{},
 		&model.ClientInbound{},
+		&model.ClientExternalLink{},
 		&model.ClientGroup{},
 		&model.InboundFallback{},
 		&model.NodeClientTraffic{},