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/schemas/protocols/inbound/amneziawg.ts

@@ -1,4 +1,5 @@
 import { z } from 'zod';
+import { OptionalClientKeepAliveSchema } from '@/schemas/primitives/keepalive';
 
 // AntD InputNumber emits null (not undefined) when the user clears it, and
 // the form store hands that null straight to safeParse on submit — a bare
@@ -23,7 +24,7 @@ export const AmneziawgClientSchema = z.object({
   publicKey: z.string().optional(),
   preSharedKey: z.string().optional(),
   allowedIPs: z.array(z.string()).default([]),
-  keepAlive: optionalClearedInt(z.number().int().min(0)),
+  keepAlive: OptionalClientKeepAliveSchema,
   forwardedPorts: z.string().default(''),
   email: z.string().min(1),
   limitIp: z.number().int().min(0).default(0),