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/migrate_data.go

@@ -47,6 +47,7 @@ func migrationModels() []any {
 		&model.InboundClientIps{},
 		&model.ClientRecord{},
 		&model.ClientInbound{},
+		&model.ClientExternalLink{},
 		&model.InboundFallback{},
 		&model.NodeClientTraffic{},
 		&model.OutboundSubscription{},