Postiz App has a High-Severity SSRF Vulnerability via Next.js

### Impact
A successful SSRF attack allows an attacker to:
- Bypass firewalls to scan and interact with internal network services/ports.
- Access sensitive cloud metadata services (e.g., AWS IMDS 169.254.169.254) to potentially leak instance credentials.
- Pivot into the internal network environment where Postiz is hosted.

### Workarounds
There are no workarounds known to this, please upgrade to Postiz version `v2.21.1`.