Improve Telegram notification formatting for Max messages

[erop39]

Summary

Adds configurable Telegram message formatting to make Max notifications easier to read when multiple users post in quick succession.

• Enhanced mode (default): stronger headers with sender name and timestamp, message body in <blockquote>, separator line when the sender changes
• Compact mode: same as enhanced, but hides repeated headers when the same user posts consecutive messages
• Plain mode: restores the previous formatting (TG_FORMAT_STYLE=plain)

Configuration

Variable	Default	Description
TG_FORMAT_STYLE	enhanced	plain, enhanced, or compact
TG_FORMAT_SEPARATOR	true	Show ━━━━━━━━━━━━━━━━ when sender changes
TG_FORMAT_TIMESTAMP	true	Show HH:MM in the header

Example (enhanced)

💬 Иван Иванов · 14:30

> Привет

━━━━━━━━━━━━━━━━
💬 Петр Сидоров · 14:31

> +1

Test plan

• pytest — 243 tests passed
• Unit tests for _format_time, _build_header, _needs_separator, MessageFormatter
• Config tests for new env vars and invalid style fallback