feat: Phase 1 Group 5 — add account_id to PSA and notification tables

psa_post_log: backfill via psa_connection, fallback to posted_by user
psa_member_mappings: backfill via psa_connection
notification_logs: backfill via notification_config

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

backend/tests/test_phase1_migrations.py

@@ -343,3 +343,41 @@ async def test_user_pinned_tree_account_id_matches_user(test_db: AsyncSession):
     result = await test_db.execute(select(UserPinnedTree).where(UserPinnedTree.id == pin.id))
     row = result.scalar_one()
     assert row.account_id == account.id
+
+
+# ── Group 5: PSA & notifications ─────────────────────────────────────────────
+
+@pytest.mark.asyncio
+async def test_psa_member_mapping_account_id_matches_connection(test_db: AsyncSession):
+    """psa_member_mappings.account_id must match psa_connection's account_id."""
+    from app.models.psa_connection import PsaConnection
+    from app.models.psa_member_mapping import PsaMemberMapping
+
+    account, user = await _make_account_and_user(test_db, "psa1")
+    conn = PsaConnection(
+        account_id=account.id,
+        provider="connectwise",
+        display_name="Test CW",
+        site_url="https://cw.example.com",
+        company_id="TEST",
+        credentials_encrypted="placeholder",
+    )
+    test_db.add(conn)
+    await test_db.flush()
+
+    mapping = PsaMemberMapping(
+        psa_connection_id=conn.id,
+        user_id=user.id,
+        account_id=account.id,
+        external_member_id="cw-123",
+        external_member_name="Test User",
+        matched_by="manual_admin",
+    )
+    test_db.add(mapping)
+    await test_db.commit()
+
+    result = await test_db.execute(
+        select(PsaMemberMapping).where(PsaMemberMapping.id == mapping.id)
+    )
+    row = result.scalar_one()
+    assert row.account_id == account.id