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.

docs/public/openapi.json

@@ -1759,9 +1759,15 @@
             "type": "string"
           },
           "keepAlive": {
-            "description": "Seconds between PersistentKeepalive packets; 0 sends none, omit to keep the stored value",
-            "nullable": true,
-            "type": "integer"
+            "description": "Numeric values or AmneziaWG ranges; 0 disables, omission preserves.",
+            "oneOf": [
+              {
+                "type": "integer"
+              },
+              {
+                "type": "string"
+              }
+            ]
           },
           "limitIp": {
             "description": "IP limit for this client",
@@ -1981,7 +1987,15 @@
             "type": "integer"
           },
           "keepAlive": {
-            "type": "integer"
+            "description": "Numeric values or AmneziaWG ranges.",
+            "oneOf": [
+              {
+                "type": "integer"
+              },
+              {
+                "type": "string"
+              }
+            ]
           },
           "limitHwid": {
             "type": "integer"