AUTHENTIK_LISTEN__TRUSTED_PROXY_CIDRS with single IPs without CIDR suffix prevents startup in 2026.8.0

### Describe the bug

After upgrading to **authentik 2026.8.0**, setting `AUTHENTIK_LISTEN__TRUSTED_PROXY_CIDRS` with a plain IP address causes authentik to fail during startup with a imprecise error:

```plain text
0: invalid IP address syntax
```

The error does not indicate that `AUTHENTIK_LISTEN__TRUSTED_PROXY_CIDRS` is the problematic setting.

This appears to be a breaking change because single IPv4 addresses have been working prior to 2026.8. However, this requirement is not listed under breaking changes in the release notes for 2026.8.

### Expected behavior

One of the following would be expected:

1. Plain IP addresses should continue to be accepted as trusted proxy entries, as in previous behavior.
2. Or, if only CIDR notation is now supported, authentik should fail with a clear validation error, for example:

```plain text
Invalid value for AUTHENTIK_LISTEN__TRUSTED_PROXY_CIDRS: "172.16.0.15" is not valid CIDR notation. Use "172.16.0.15/32" for a single IPv4 address.
```

Additionally, if this is an intentional change, it should be documented as a breaking change in the release notes.

---

### Fix / Workaround

Changing the plain IP address to explicit CIDR notation fixes the issue:

```yaml
AUTHENTIK_LISTEN__TRUSTED_PROXY_CIDRS: 127.0.0.0/8, 172.16.0.15/32
```

After this change, the server starts successfully.

### How to reproduce

Set env for AUTHENTIK_LISTEN__TRUSTED_PROXY_CIDRS to a singe ip without CIDR suffix
```yaml
AUTHENTIK_LISTEN__TRUSTED_PROXY_CIDRS: 127.0.0.0/8, 172.16.0.15
```

### Expected behavior

One of the following would be expected:

1. Plain IP addresses should continue to be accepted as trusted proxy entries, as in previous behavior.
2. Or, if only CIDR notation is now supported, authentik should fail with a clear validation error, for example:

```plain text
Invalid value for AUTHENTIK_LISTEN__TRUSTED_PROXY_CIDRS: "172.16.0.15" is not valid CIDR notation. Use "172.16.0.15/32" for a single IPv4 address.
```

Additionally, if this is an intentional change, it should be documented as a breaking change in the release notes.

### Screenshots

_No response_

### Additional context

_No response_

### Deployment Method

Docker

### Version

2026.8.0

### Relevant log output

```shell
0: invalid IP address syntax
```