Add contact backup with full metadata and active avatars

[diogogithub]

Summary

• add tg_contacts_backup.py to export Telegram contacts as CSV and JSON;
• retrieve detailed per-contact metadata with contacts.GetContactsRequest and users.GetFullUserRequest;
• download each contact's current active profile photo by default;
• respect Telegram flood waits and preserve per-contact errors without aborting the export;
• document privacy implications, output structure, and command options;
• ignore Telethon session credentials and generated archives;
• constrain dependencies to the compatible Telethon 1.x API;
• add basic Python CI checks and correct the MIT license attribution.

Checks

• python -m py_compile tg_contacts_backup.py
• CI validates both scripts on Python 3.10 and 3.12.

Feature request addressed

This implements the Codeberg request to preserve contact IDs, names, usernames, richer contact information, and active avatars for account recovery scenarios.