v1.8.4

## Security

Three vulnerabilities reported by CERT-EU Offensive Security Team via coordinated disclosure:

- **Remote Code Execution via Server-Side Template Injection** (CVSS 9.2 Critical)
  - User-created transformation prompts were rendered by an unsandboxed Jinja2 environment, allowing arbitrary Python code execution on the server. Bumped ai-prompter to 0.4.0 which uses `SandboxedEnvironment` for all template rendering.

- **Arbitrary file write via path traversal** (CVSS 7.0 High)
  - File upload did not sanitize filenames, allowing path traversal payloads (e.g., `../../../../tmp/test.txt`) to write files outside the upload directory. Filenames are now sanitized with `os.path.basename()` and resolved paths are validated.

- **Arbitrary file read via Local File Inclusion** (CVSS 8.2 High)
  - The source creation endpoint accepted arbitrary `file_path` values, allowing reading of any file on the container (e.g., `/etc/passwd`, `/proc/self/environ`). File paths are now validated to be within the uploads directory.

### Affected versions

All versions up to and including v1.8.3.

### Recommended action

Upgrade to v1.8.4 immediately.

### Credit

Reported by [CERT-EU](https://cert.europa.eu) Offensive Security Team via coordinated vulnerability disclosure.