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/web/service/client_link.go

@@ -55,7 +55,9 @@ func applyClientRecordMerge(row *model.ClientRecord, incoming *model.ClientRecor
 		row.AllowedIPs = incoming.AllowedIPs
 	}
 	row.PreSharedKey = incoming.PreSharedKey
-	row.KeepAlive = incoming.KeepAlive
+	if incoming.KeepAlive != "" {
+		row.KeepAlive = incoming.KeepAlive
+	}
 	row.SubID = incoming.SubID
 	row.LimitIP = incoming.LimitIP
 	row.TotalGB = incoming.TotalGB