Merged outbound+inbound conversation threads.

Returns a list of threads, each thread = one publisher (or one stranger email when no publisher match) with all messages from both directions sorted oldest-first inside the thread. The thread list itself is sorted by last_message_at descending so the most-recently-active conversation lands at the top.

Includes the cold-outreach invite that opened the conversation, any auto follow-up sends, and every inbound reply tied to the same publisher / address.