[BUG] Gotify fallback webhook URL is missing token placeholder

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Current Behavior

### Describe the bug

`AlerterProperties.java` defines the fallback default Gotify webhook URL as:

```java
private String gotifyWebhookUrl = "https://push.example.de/message?token=";
```

But `GotifyAlertNotifyHandlerImpl` formats this value with the receiver token:

```java
String webHookUrl = String.format(alerterProperties.getGotifyWebhookUrl(), receiver.getGotifyToken());
```

Because the fallback default does not contain a `%s` placeholder, `String.format()` ignores `receiver.getGotifyToken()`. If this Java fallback value is used, the generated URL becomes:

```text
https://push.example.de/message?token=
```

instead of including the Gotify token.

The configured value in `application.yml` already uses the expected format:

```yaml
gotify-webhook-url: http://127.0.0.1/message?token=%s
```

So the Java fallback default should be consistent with the configured default.

### Expected Behavior

The fallback default should include the token placeholder:

```java
private String gotifyWebhookUrl = "https://push.example.de/message?token=%s";
```

### Steps To Reproduce

_No response_

### Environment

```markdown
HertzBeat master branch / 2.0-SNAPSHOT.
```

### Debug logs

_No response_

### Anything else?

_No response_

Start by comparing the fallback value in AlerterProperties.java with the formatting call in GotifyAlertNotifyHandlerImpl. Confirm that the fallback URL contains the token placeholder and that formatting it with a receiver token produces the expected Gotify URL.

Written by the indexing model from the issue text.