feat(04-01): add Wave 0 xfail stubs for FOLD and SHARE requirements

- test_folders.py: 13 stubs for FOLD-01..05 (create, rename, delete, move, breadcrumb, sort, FTS)
- test_shares.py: 7 stubs for SHARE-01..05 (share, received list, quota isolation, revoke, duplicate)
- FTS tests carry both xfail and skipif(INTEGRATION) decorators
- All stubs: xfail(strict=False), body is pytest.xfail("not implemented yet")

backend/tests/test_shares.py

@@ -0,0 +1,79 @@
+"""
+Share API tests — Wave 0 xfail stubs for Phase 4.
+
+All tests in this file are xfail stubs. They will be implemented in Plan 04-05.
+The stubs ensure pytest collects them and keeps CI green before implementation
+code exists.
+"""
+from __future__ import annotations
+
+import os
+
+import pytest
+
+
+# ---------------------------------------------------------------------------
+# SHARE-01: Share a document
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.xfail(strict=False)
+async def test_share_success(async_client, auth_user):
+    """POST /api/shares grants share; recipient can see doc via GET /api/shares/received."""
+    pytest.xfail("not implemented yet")
+
+
+@pytest.mark.xfail(strict=False)
+async def test_share_handle_not_found(async_client, auth_user):
+    """POST /api/shares with unknown handle returns 404."""
+    pytest.xfail("not implemented yet")
+
+
+# ---------------------------------------------------------------------------
+# SHARE-02: List shared-with-me
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.xfail(strict=False)
+async def test_shared_with_me(async_client, auth_user):
+    """GET /api/shares/received lists docs shared with current user."""
+    pytest.xfail("not implemented yet")
+
+
+# ---------------------------------------------------------------------------
+# SHARE-03: Quota isolation
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.xfail(strict=False)
+async def test_share_no_quota_impact(async_client, auth_user):
+    """Share does not increment recipient's quota used_bytes."""
+    pytest.xfail("not implemented yet")
+
+
+# ---------------------------------------------------------------------------
+# SHARE-04: Revoke share
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.xfail(strict=False)
+async def test_revoke_share(async_client, auth_user):
+    """DELETE /api/shares/{id} removes share; GET /api/shares/received no longer lists the doc."""
+    pytest.xfail("not implemented yet")
+
+
+@pytest.mark.xfail(strict=False)
+async def test_share_revoke_wrong_owner_404(async_client, auth_user):
+    """DELETE /api/shares/{id} by non-owner returns 404."""
+    pytest.xfail("not implemented yet")
+
+
+# ---------------------------------------------------------------------------
+# SHARE-05: Duplicate share
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.xfail(strict=False)
+async def test_share_duplicate(async_client, auth_user):
+    """POST /api/shares same doc+recipient twice returns 409."""
+    pytest.xfail("not implemented yet")