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/content/docs/en/config/amneziawg.mdx

@@ -124,6 +124,14 @@ inbound. There is no separate toggle for this: unlike a kernel tunnel,
 there's no other way for a peer's traffic to reach the internet once it's
 decapsulated.
 
+## Keepalive ranges
+
+AmneziaWG clients accept `keepAlive` as a number or an ordered range from 0 to 65535, such as `15-25`. Omitted values keep the existing setting; explicit `0` disables keepalive. Native `.conf` files preserve ranges, while Clash/Mihomo scalar-only output uses the lower bound (`0-25` therefore disables keepalive). Plain WireGuard clients support only a scalar value. Upgrade sub-nodes before sending them range values because older nodes parse keepalive as an integer.
+
+Parser support is present in the native tools and the panel embedded engine: [the native `awg-tools` v3.1.20260812 config parser](https://github.com/amnezia-vpn/amneziawg-tools/blob/v3.1.20260812/src/config.c#L596-L599) sends `PersistentKeepalive` to its [ordered-range parser](https://github.com/amnezia-vpn/amneziawg-tools/blob/v3.1.20260812/src/type.c#L40-L59), and this panel pins [amneziawg-go v3.1.20260828](https://github.com/amnezia-vpn/amneziawg-go/tree/v3.1.20260828) in `go.mod`; its [UAPI parser](https://github.com/amnezia-vpn/amneziawg-go/blob/v3.1.20260828/device/uapi.go#L663-L674) reads the interval as a `UintRange`. This confirms parser support for these code paths, not interoperability with every third-party client. The panel enforces the documented 0-65535 limit on new values; the native parser's syntax support is not a substitute for that validation.
+
+![AmneziaWG client editor with an ordered keepalive range](/images/amneziawg-keepalive-range.png)
+
 ## What the configuration looks like
 
 A client's downloadable `.conf` (also what the `vpn://` share link encodes,

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"