feat(amneziawg): support persistent keepalive ranges (#6881)

AmneziaWG clients accept a randomised PersistentKeepalive range such as 15-25,
which makes keepalive timing harder to fingerprint; the panel only stored a
single number. A client's keepalive may now be a range, validated the way
AmneziaWG parses it. The downloadable .conf and vpn:// profile carry the range,
Clash/Mihomo output gets its lower bound, and plain WireGuard stays scalar-only.
The range lives in an additive wg_keep_alive_range column, so the integer
column and an older panel binary keep working after a rollback.

internal/sub/clash_service.go

@@ -652,7 +652,7 @@ func (s *SubClashService) buildWireguardProxy(subReq *SubService, inbound *model
 	if client.PreSharedKey != "" {
 		proxy["pre-shared-key"] = client.PreSharedKey
 	}
-	if ka := client.KeepAliveSeconds(); ka > 0 {
+	if ka, ok := keepAliveForScalarExport(client); ok {
 		proxy["persistent-keepalive"] = ka
 	}
 	for _, addr := range client.AllowedIPs {
@@ -794,7 +794,7 @@ func (s *SubClashService) buildAmneziaWGProxy(subReq *SubService, inbound *model
 	if client.PreSharedKey != "" {
 		proxy["pre-shared-key"] = client.PreSharedKey
 	}
-	if ka := client.KeepAliveSeconds(); ka > 0 {
+	if ka, ok := keepAliveForScalarExport(client); ok {
 		proxy["persistent-keepalive"] = ka
 	}