docs: activity log — backend v2.8.74 (chat uuid→session ObjectId participant fix)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

09 - Audits/Activity Log.md

@@ -1123,4 +1123,20 @@ visible under their fields instead of only in the console.
 
 ---
 
+### 2026-06-04 — backend v2.8.74 — chat: map uuid participants → session ObjectId (buyer↔seller fix)
+
+Root cause confirmed from logs: a buyer opens a chat "with seller X" using X's
+marketplace **Postgres uuid** (669c0dac-…), but X's session/JWT id is their
+**legacy ObjectId** (e0527…). Conversations key participants by the session id,
+so buyer and seller landed in two separate direct threads (each saw only their
+own messages). ChatService.createChat now normalises any uuid participant (and
+createdBy) → users.legacy_object_id via a direct PG lookup BEFORE dedup/create —
+resolving TOWARD the session-id format (the reverted v2.8.72 resolved toward uuid
+and broke membership). Both sides now converge on one conversation + socket room.
+Note: pre-existing mismatched conversations are orphaned; a freshly opened chat
+dedups correctly for both parties.
+**Verification:** backend tsc clean.
+
+---
+
 <!-- Add new entries above this line. Newest at top. -->