fix(clients): let an explicit 0 actually disable PersistentKeepalive

Addresses review feedback on the previous commit.

UpdateInboundClient carries a stored keepalive forward whenever the incoming
one is zero, so the settings JSON and the running peer survive a metadata-only
edit that omits the field. That was a 0 -> 0 no-op while no UI could set a
nonzero value. Now that the client form can, the carry-forward became reachable
in the other direction: a client created at the form's default of 25 could
never be returned to 0, and the hint text shipped to all 13 locales -- "0
disables it" -- described something the backend silently refused. The save even
reported success, because a settings blob that came back byte-identical skips
the transaction entirely.

The zero value cannot carry that distinction, so model.Client.KeepAlive becomes
*int: nil means the field was never sent, &0 means "send no keepalives". The
pointer survives the internal marshal in ClientService.Update, which is where an
explicit 0 was being erased by omitempty before UpdateInboundClient ever saw it.
ClientRecord.KeepAlive stays a plain int -- it is the stored column, where
"unset" has no meaning -- and the conversions bridge the two.

Two tests, both red before this change in the direction they cover: an explicit
0 must reach wg_keep_alive, and an update that omits the field must still leave
a stored 25 alone.

Also adds the output transform every other numeric field in the client form
already has, so a cleared box sends 0 rather than null.

internal/sub/service.go

@@ -677,8 +677,8 @@ func (s *SubService) genWireguardLink(inbound *model.Inbound, email string) stri
 	if client.PreSharedKey != "" {
 		params["presharedkey"] = client.PreSharedKey
 	}
-	if client.KeepAlive > 0 {
-		params["keepalive"] = strconv.Itoa(client.KeepAlive)
+	if ka := client.KeepAliveSeconds(); ka > 0 {
+		params["keepalive"] = strconv.Itoa(ka)
 	}
 	return buildLinkWithParams(link, params, s.genRemark(inbound, email, "", ""))
 }
@@ -779,8 +779,8 @@ func amneziaWGConfigText(server *amneziawg.ServerSettings, client *model.Client,
 	}
 	b.WriteString("AllowedIPs = 0.0.0.0/0, ::/0\n")
 	fmt.Fprintf(&b, "Endpoint = %s:%d", host, port)
-	if client.KeepAlive > 0 {
-		fmt.Fprintf(&b, "\nPersistentKeepalive = %d", client.KeepAlive)
+	if ka := client.KeepAliveSeconds(); ka > 0 {
+		fmt.Fprintf(&b, "\nPersistentKeepalive = %d", ka)
 	}
 
 	return b.String()