fix: use correlated subquery in psa_post_log backfill to avoid invalid FROM-clause reference

PostgreSQL UPDATE...FROM does not allow the updated table to be
referenced inside the FROM clause's JOIN conditions. Replace the
LEFT JOIN psa_connections with a correlated subquery.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/alembic/versions/8aac5b372402_add_account_id_psa_notifications.py

@@ -25,11 +25,15 @@ def upgrade() -> None:
 
     # Step 2: BACKFILL
     # psa_post_log: prefer psa_connection → fallback to posted_by user
+    # Note: cannot reference the updated table (ppl) inside the FROM clause JOIN,
+    # so use a correlated subquery for psa_connections lookup instead.
     op.execute("""
         UPDATE psa_post_log ppl
-        SET account_id = COALESCE(pc.account_id, u.account_id)
+        SET account_id = COALESCE(
+            (SELECT account_id FROM psa_connections WHERE id = ppl.psa_connection_id),
+            u.account_id
+        )
         FROM users u
-        LEFT JOIN psa_connections pc ON pc.id = ppl.psa_connection_id
         WHERE ppl.posted_by = u.id
           AND ppl.account_id IS NULL
     """)