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/zod.ts

@@ -441,7 +441,7 @@ export const ClientSchema = z.object({
   forwardedPorts: z.string().optional(),
   group: z.string().optional(),
   id: z.string().optional(),
-  keepAlive: z.number().int().nullable().optional(),
+  keepAlive: z.union([z.number().int(), z.string()]).optional(),
   limitIp: z.number().int(),
   password: z.string().optional(),
   preSharedKey: z.string().optional(),
@@ -495,7 +495,7 @@ export const ClientRecordSchema = z.object({
   forwardedPorts: z.string(),
   group: z.string(),
   id: z.number().int(),
-  keepAlive: z.number().int(),
+  keepAlive: z.union([z.number().int(), z.string()]),
   limitHwid: z.number().int(),
   limitIp: z.number().int(),
   password: z.string(),