Add MAIN_BOT_SESSION config to support mainbot.session file

Fix: Auto-fallback to default when DATABASE_BACKUP_MSG_ID is negative (chat ID)

Improve error message for DATABASE_BACKUP_MSG_ID validation

Add validation for DATABASE_BACKUP_MSG_ID to prevent 'int too big to convert' error

Support BOT_SESSIONS with BOT_TOKENS - use session files if provided

Fix: Use BOT_TOKENS - Pyrogram handles session files automatically

Use bot.session files instead of bot tokens

Initial commit