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/sub/service.go

@@ -147,8 +147,12 @@ func (s *SubService) GetSubs(subId string, host string) ([]string, []string, int
 	if err != nil {
 		return nil, nil, 0, traffic, err
 	}
+	externalLinks, err := s.getClientExternalLinksBySubId(subId)
+	if err != nil {
+		return nil, nil, 0, traffic, err
+	}
 
-	if len(inbounds) == 0 {
+	if len(inbounds) == 0 && len(externalLinks) == 0 {
 		return nil, nil, 0, traffic, nil
 	}
 
@@ -178,6 +182,18 @@ func (s *SubService) GetSubs(subId string, host string) ([]string, []string, int
 			seenEmails[client.Email] = struct{}{}
 		}
 	}
+	for _, ext := range externalLinks {
+		if ext.Enable {
+			hasEnabledClient = true
+		}
+		for _, el := range expandEntry(ext) {
+			if link := applyRemarkToLink(el.Link, el.Name); link != "" {
+				result = append(result, link)
+				emails = append(emails, ext.Email)
+				seenEmails[ext.Email] = struct{}{}
+			}
+		}
+	}
 
 	uniqueEmails := make([]string, 0, len(seenEmails))
 	for e := range seenEmails {