ci: announce stable releases in Telegram

[xPROMSx]

Summary

Post new stable GitHub Releases from Telegram Web Proxy Manager to the shared xPROMSx | Open Source channel.

• Trigger only on release.published, skipping drafts and prereleases.
• Format concise bilingual Russian/English announcements with version, up to three release-note highlights, and a direct GitHub Release link.
• Provide workflow_dispatch to send an explicitly requested test announcement.
• Use the official Telegram Bot API directly with Python's standard library (no third-party GitHub Actions or dependencies).
• Use only the repository secret TELEGRAM_BOT_TOKEN; no token in code or logs.
• Restrict GITHUB_TOKEN to contents: read; do not change existing CI or production scripts.

Owner setup

1. Create one bot with @Botfather and grant it only Post Messages administrator permission in @promsdev.
2. Save its token as TELEGRAM_BOT_TOKEN in this repository's Settings → Secrets and variables → Actions.
3. After merge, optionally run Telegram release announcements manually to send a clearly labeled test message.
4. Subsequent stable releases are posted automatically.

Note: Release events created with GitHub's ephemeral GITHUB_TOKEN do not trigger new workflows. Releases created through GitHub's UI normally do.

No production changes, no bot token and no actual Telegram messages are included in this PR.