feat(auth): enforce owner_id across 2.0-rc persistence layer

Add request-scoped contextvar-based owner filtering to threads_meta,
runs, run_events, and feedback repositories. Router code is unchanged
— isolation is enforced at the storage layer so that any caller that
forgets to pass owner_id still gets filtered results, and new routes
cannot accidentally leak data.

Core infrastructure
-------------------
- deerflow/runtime/user_context.py (new):
  - ContextVar[CurrentUser | None] with default None
  - runtime_checkable CurrentUser Protocol (structural subtype with .id)
  - set/reset/get/require helpers
  - AUTO sentinel + resolve_owner_id(value, method_name) for sentinel
    three-state resolution: AUTO reads contextvar, explicit str
    overrides, explicit None bypasses the filter (for migration/CLI)

Repository changes
------------------
- ThreadMetaRepository: create/get/search/update_*/delete gain
  owner_id=AUTO kwarg; read paths filter by owner, writes stamp it,
  mutations check ownership before applying
- RunRepository: put/get/list_by_thread/delete gain owner_id=AUTO kwarg
- FeedbackRepository: create/get/list_by_run/list_by_thread/delete
  gain owner_id=AUTO kwarg
- DbRunEventStore: list_messages/list_events/list_messages_by_run/
  count_messages/delete_by_thread/delete_by_run gain owner_id=AUTO
  kwarg. Write paths (put/put_batch) read contextvar softly: when a
  request-scoped user is available, owner_id is stamped; background
  worker writes without a user context pass None which is valid
  (orphan row to be bound by migration)

Schema
------
- persistence/models/run_event.py: RunEventRow.owner_id = Mapped[
  str | None] = mapped_column(String(64), nullable=True, index=True)
- No alembic migration needed: 2.0 ships fresh, Base.metadata.create_all
  picks up the new column automatically

Middleware
----------
- auth_middleware.py: after cookie check, call get_optional_user_from_
  request to load the real User, stamp it into request.state.user AND
  the contextvar via set_current_user, reset in a try/finally. Public
  paths and unauthenticated requests continue without contextvar, and
  @require_auth handles the strict 401 path

Test infrastructure
-------------------
- tests/conftest.py: @pytest.fixture(autouse=True) _auto_user_context
  sets a default SimpleNamespace(id="test-user-autouse") on every test
  unless marked @pytest.mark.no_auto_user. Keeps existing 20+
  persistence tests passing without modification
- pyproject.toml [tool.pytest.ini_options]: register no_auto_user
  marker so pytest does not emit warnings for opt-out tests
- tests/test_user_context.py: 6 tests covering three-state semantics,
  Protocol duck typing, and require/optional APIs
- tests/test_thread_meta_repo.py: one test updated to pass owner_id=
  None explicitly where it was previously relying on the old default

Test results
------------
- test_user_context.py: 6 passed
- test_auth*.py + test_langgraph_auth.py + test_ensure_admin.py: 127
- test_run_event_store / test_run_repository / test_thread_meta_repo
  / test_feedback: 92 passed
- Full backend suite: 1905 passed, 2 failed (both @requires_llm flaky
  integration tests unrelated to auth), 1 skipped

backend/tests/conftest.py

@@ -6,8 +6,11 @@ issues when unit-testing lightweight config/registry code in isolation.
 
 import sys
 from pathlib import Path
+from types import SimpleNamespace
 from unittest.mock import MagicMock
 
+import pytest
+
 # Make 'app' and 'deerflow' importable from any working directory
 sys.path.insert(0, str(Path(__file__).parent.parent))
 
@@ -31,3 +34,44 @@ _executor_mock.MAX_CONCURRENT_SUBAGENTS = 3
 _executor_mock.get_background_task_result = MagicMock()
 
 sys.modules["deerflow.subagents.executor"] = _executor_mock
+
+
+# ---------------------------------------------------------------------------
+# Auto-set user context for every test unless marked no_auto_user
+# ---------------------------------------------------------------------------
+#
+# Repository methods read ``owner_id`` from a contextvar by default
+# (see ``deerflow.runtime.user_context``). Without this fixture, every
+# pre-existing persistence test would raise RuntimeError because the
+# contextvar is unset. The fixture sets a default test user on every
+# test; tests that explicitly want to verify behaviour *without* a user
+# context should mark themselves ``@pytest.mark.no_auto_user``.
+
+
+@pytest.fixture(autouse=True)
+def _auto_user_context(request):
+    """Inject a default ``test-user-autouse`` into the contextvar.
+
+    Opt-out via ``@pytest.mark.no_auto_user``. Uses lazy import so that
+    tests which don't touch the persistence layer never pay the cost
+    of importing runtime.user_context.
+    """
+    if request.node.get_closest_marker("no_auto_user"):
+        yield
+        return
+
+    try:
+        from deerflow.runtime.user_context import (
+            reset_current_user,
+            set_current_user,
+        )
+    except ImportError:
+        yield
+        return
+
+    user = SimpleNamespace(id="test-user-autouse", email="test@local")
+    token = set_current_user(user)
+    try:
+        yield
+    finally:
+        reset_current_user(token)