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
534511bebb