chore: add SECURITY.md with vulnerability disclosure policy

[kirich1409]

Context

The repository has no SECURITY.md. GitHub uses this file to display a "Report a vulnerability" button in the Security tab and to direct security researchers to the correct contact.

Acceptance criteria

• Create SECURITY.md in the repo root
• Specify supported versions (e.g., only latest release)
• Provide a private disclosure channel (GitHub private vulnerability reporting or email)
• Keep it concise — 1 page max

Template

GitHub's recommended minimal structure:

## Supported Versions

| Version | Supported |
|---------|-----------|
| 1.x     | ✅        |

## Reporting a Vulnerability

Please use [GitHub private vulnerability reporting](https://github.com/AndroidBroadcast/Featured/security/advisories/new)
or email kirill@androidbroadcast.dev.

Do not open a public issue for security vulnerabilities.