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/app/gateway/auth_middleware.py

@@ -1,6 +1,11 @@
 """Global authentication middleware — fail-closed safety net.
 
-Rejects unauthenticated requests to non-public paths with 401.
+Rejects unauthenticated requests to non-public paths with 401. When a
+request passes the cookie check, resolves the JWT payload to a real
+``User`` object and stamps it into both ``request.state.user`` and the
+``deerflow.runtime.user_context`` contextvar so that repository-layer
+owner filtering works automatically via the sentinel pattern.
+
 Fine-grained permission checks remain in authz.py decorators.
 """
 
@@ -12,6 +17,7 @@ from starlette.responses import JSONResponse
 from starlette.types import ASGIApp
 
 from app.gateway.auth.errors import AuthErrorCode
+from deerflow.runtime.user_context import reset_current_user, set_current_user
 
 # Paths that never require authentication.
 _PUBLIC_PATH_PREFIXES: tuple[str, ...] = (
@@ -68,4 +74,22 @@ class AuthMiddleware(BaseHTTPMiddleware):
                 },
             )
 
-        return await call_next(request)
+        # Resolve the full user now so repository-layer owner filters
+        # can read from the contextvar. We use the "optional" flavour so
+        # middleware never raises on bad tokens — the cookie-presence
+        # check above plus the @require_auth decorator provide the
+        # strict gates. A stale/invalid token yields user=None here;
+        # the request continues without a contextvar, and any protected
+        # endpoint will still be rejected by @require_auth.
+        from app.gateway.deps import get_optional_user_from_request
+
+        user = await get_optional_user_from_request(request)
+        if user is None:
+            return await call_next(request)
+
+        request.state.user = user
+        token = set_current_user(user)
+        try:
+            return await call_next(request)
+        finally:
+            reset_current_user(token)