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/keepalive.go

@@ -0,0 +1,18 @@
+package sub
+
+import (
+	"strings"
+
+	"github.com/mhsanaei/3x-ui/v3/internal/database/model"
+)
+
+func keepAliveForScalarExport(client model.Client) (int, bool) {
+	if client.KeepAlive == nil {
+		return 0, false
+	}
+	if strings.Contains(client.KeepAliveText(), "-") && client.KeepAlive.Validate(true) == nil {
+		return client.KeepAlive.MinSeconds(), true
+	}
+	value := client.KeepAliveSeconds()
+	return value, value > 0
+}