Add /muted digest for buffered muted chats

[erop39]

Summary

• added MutedMessageBuffer to collect messages from muted Max chats instead of dropping them
• added /muted and /readmuted Telegram commands plus 📭 Заглушённые callback button to flush buffered messages grouped by chat
• refactored single-message forwarding into reusable forward_message(...) and reused it in muted digest flushing
• wired MUTED_DIGEST_ENABLED config through startup, polling, and status messages
• updated docs and .env.example with the new configuration
• added tests for muted buffer, muted digest, and new Telegram handler flows

Validation

• python3 -m pytest -q (227 passed)