Add /muted command and muted chat digest button

[erop39]

Summary

Adds support for reading buffered messages from muted Max chats via /muted (alias /readmuted) and an inline 📭 Заглушённые button.

What changed

• MUTED_DIGEST_ENABLED=true — enables muted-chat buffering and Telegram polling for digest commands (even when REPLY_ENABLED=false)
• Messages from muted Max chats are stored in an in-memory buffer instead of being silently dropped
• /muted or /readmuted — flushes buffered messages grouped by chat
• 📭 Заглушённые button appears on startup/reconnect notifications when digest mode is enabled
• Buffer is pruned when messages are read in Max (via read marks)

New files

• app/muted_buffer.py — in-memory message buffer grouped by chat
• app/muted_digest.py — flush logic for buffered muted messages
• Tests for buffer, digest, and handler integration

How to enable

Add to .env:

MUTED_DIGEST_ENABLED=true

Optionally also set SKIP_MUTED=true (digest mode automatically enables skip-muted behavior).

Register /muted in BotFather if desired — the handler is now registered in code.

Testing

• All 227 tests pass, including new tests for muted buffer/digest/handlers