docs: activity log — backend v2.8.70 (persist telegram user updates on sign-in)

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

09 - Audits/Activity Log.md

@@ -1057,4 +1057,20 @@ reviewed" handled gracefully.
 
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+### 2026-06-03 — backend v2.8.70 — persist telegram user updates on sign-in (new-user login)
+
+Investigating "new Telegram account / new device can't log in (existing works)".
+telegramAuth mutates the user after the TelegramLink lookup (telegramVerified
+flip, lazily backfilled referralCode, lastLoginAt) but only NEW users hit
+user.save(); existing/half-created users were never re-persisted → a backfilled
+referralCode never landed and subsequent sign-ins kept tripping the non-sparse
+users.referralCode index. Added a best-effort user.save() before linking.
+**Note:** background logs in the user's screenshots show "Backend 2.6.49" — the
+deployed dev backend may be far behind (none of the 2.8.x auth/email/chat fixes
+live). Asked the user to confirm/redeploy the backend and share the exact error
+the new device shows (frontend surfaces backend message verbatim).
+**Verification:** backend tsc clean.
+
+---
+
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