archiver-bot: reply in threads
Make every bot response a thread reply instead of a fresh top-level
message. send_notice gains a thread_root kwarg that adds m.relates_to
with rel_type=m.thread plus the m.in_reply_to fallback. on_message
picks the thread root via thread_root_for(): if the user's message is
already inside a thread we reply into that one, otherwise we start a
new thread rooted on their message.
For Radarr/Sonarr download notifications to thread back to the
original request, tracked_requests.json now stores the request's
event_id alongside the room. load_tracked normalizes legacy bare-
string entries so existing tracked items keep working (just without
threading on the eventual notification).