fix(sub): keep the client identity on every subscription link (#6098)

876d55f2 put EMAIL/USERNAME in the same first-link-only bucket as the usage
tokens, so a client attached to several inbounds got its email on whichever
inbound sorted first and bare inbound names on all the rest. With the shipped
default template ({{INBOUND}}-{{EMAIL}}|...) that makes every profile after
the first indistinguishable between clients — the point of the token.

The two are not alike: the usage block repeats identical numbers on every
link, while the identity is what tells one imported profile from another.
Restore identity on all body links and leave usage first-link-only.

Reported again in #6029 and #5659, which asked for the same revert.

internal/sub/remark_vars.go

@@ -481,15 +481,6 @@ var connectionTokens = map[string]bool{
 
 var displayRemoveTokens = mergeTokenSets(usageInfoTokens, connectionTokens)
 
-// firstLinkOnlyBodyTokens are stripped from every subscription-body link after a
-// client's first one: the usage/info tokens plus the per-client EMAIL/USERNAME
-// identity. A client app needs the email once, so repeating it on every link of
-// the same subscription is noise — show it on the first link only, like traffic.
-var firstLinkOnlyBodyTokens = mergeTokenSets(usageInfoTokens, map[string]bool{
-	"EMAIL":    true,
-	"USERNAME": true,
-})
-
 func mergeTokenSets(sets ...map[string]bool) map[string]bool {
 	out := make(map[string]bool)
 	for _, set := range sets {
@@ -560,7 +551,7 @@ func (s *SubService) effectiveTemplate(email string) string {
 		s.usageShown = map[string]bool{}
 	}
 	if s.usageShown[email] {
-		return filterRemarkTemplate(translated, firstLinkOnlyBodyTokens)
+		return filterRemarkTemplate(translated, usageInfoTokens)
 	}
 	s.usageShown[email] = true
 	return translated