feat(matrix): Tier 1 enhancement — reactions, read receipts, rich formatting, room management

Cherry-picked from PR #4338 by nepenth, resolved against current main.

Adds:
- Processing lifecycle reactions (eyes/checkmark/cross) via MATRIX_REACTIONS env
- Reaction send/receive with ReactionEvent + UnknownEvent fallback for older nio
- Fire-and-forget read receipts on text and media messages
- Message redaction, room history fetch, room creation, user invite
- Presence status control (online/offline/unavailable)
- Emote (/me) and notice message types with HTML rendering
- XSS-hardened markdown-to-HTML converter (strips raw HTML preprocessor,
  sanitizes link URLs against javascript:/data:/vbscript: schemes)
- Comprehensive regex fallback with full block/inline markdown support
- Markdown>=3.6 added to [matrix] extras in pyproject.toml
- 46 new tests covering all features and security hardening

pyproject.toml

@@ -43,7 +43,7 @@ dev = ["debugpy>=1.8.0,<2", "pytest>=9.0.2,<10", "pytest-asyncio>=1.3.0,<2", "py
 messaging = ["python-telegram-bot>=22.6,<23", "discord.py[voice]>=2.7.1,<3", "aiohttp>=3.13.3,<4", "slack-bolt>=1.18.0,<2", "slack-sdk>=3.27.0,<4"]
 cron = ["croniter>=6.0.0,<7"]
 slack = ["slack-bolt>=1.18.0,<2", "slack-sdk>=3.27.0,<4"]
-matrix = ["matrix-nio[e2e]>=0.24.0,<1"]
+matrix = ["matrix-nio[e2e]>=0.24.0,<1", "Markdown>=3.6,<4"]
 cli = ["simple-term-menu>=1.0,<2"]
 tts-premium = ["elevenlabs>=1.0,<2"]
 voice = [