Previously only handled ticket_created. Now handles:
- status_changed: shows old → new status with actor name
- comment_added: shows author + 200-char preview (opt-in via MATRIX_NOTIFY_COMMENTS)
- mention: targeted notification when @username used in comment
- assigned: shows new assignee + actor (opt-in via MATRIX_NOTIFY_ASSIGNMENTS)
Unknown events fall back to a debug line rather than being silently dropped.
Avatar updated to ticket emoji via Synapse admin API (mxc already applied live).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>