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.

frontend/src/generated/examples.ts

@@ -424,7 +424,7 @@ export const EXAMPLES: Record<string, unknown> = {
     "forwardedPorts": "",
     "group": "",
     "id": "",
-    "keepAlive": null,
+    "keepAlive": 0,
     "limitIp": 0,
     "password": "",
     "preSharedKey": "",