feat: Phase 1 Group 4 — add account_id to user_folders and user_pinned_trees

Backfill: user_id → users.account_id

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

backend/tests/test_phase1_migrations.py

@@ -298,3 +298,48 @@ async def test_step_usage_log_account_id_is_logger_account(test_db: AsyncSession
     assert row.account_id == account.id, (
         f"account_id should be logger's account ({account.id}), got {row.account_id}"
     )
+
+
+# ── Group 4: User personalization ────────────────────────────────────────────
+
+@pytest.mark.asyncio
+async def test_user_folder_account_id_matches_user(test_db: AsyncSession):
+    """user_folders.account_id must match the owning user's account_id."""
+    from app.models.folder import UserFolder
+
+    account, user = await _make_account_and_user(test_db, "uf1")
+    folder = UserFolder(
+        user_id=user.id,
+        account_id=account.id,
+        name="My Folder",
+        color="#6366f1",
+        icon="folder",
+        display_order=0,
+    )
+    test_db.add(folder)
+    await test_db.commit()
+
+    result = await test_db.execute(select(UserFolder).where(UserFolder.id == folder.id))
+    row = result.scalar_one()
+    assert row.account_id == account.id
+
+
+@pytest.mark.asyncio
+async def test_user_pinned_tree_account_id_matches_user(test_db: AsyncSession):
+    """user_pinned_trees.account_id must match the pinning user's account_id."""
+    from app.models.user_pinned_tree import UserPinnedTree
+
+    account, user = await _make_account_and_user(test_db, "pt1")
+    tree = await _make_tree(test_db, account, user)
+    pin = UserPinnedTree(
+        user_id=user.id,
+        tree_id=tree.id,
+        account_id=account.id,
+        display_order=0,
+    )
+    test_db.add(pin)
+    await test_db.commit()
+
+    result = await test_db.execute(select(UserPinnedTree).where(UserPinnedTree.id == pin.id))
+    row = result.scalar_one()
+    assert row.account_id == account.id