feat(persistence): PR6 T6.1 — ThreadMetaRepository.create workspace_id sentinel

`create()` now accepts `workspace_id: str | None | _AutoSentinel = AUTO`
on both the SQL and in-memory implementations (and the abstract base).
AUTO resolves via `resolve_workspace_id()` from the workspace contextvar
that PR4 AuthMiddleware sets; explicit None bypasses for migration paths;
explicit str overrides the contextvar.

Test infrastructure:
- conftest gains an autouse `_auto_workspace_context` fixture mirroring
  the existing user fixture. Opt-out via `@pytest.mark.no_auto_workspace`.
- A SQLAlchemy `after_create` listener on `Base.metadata` seeds the
  matching `test-workspace-autouse` + `test-user-autouse` rows whenever
  `init_engine` runs `create_all()`, so the FK from threads_meta to
  workspaces resolves. Alembic migration tests bypass create_all and are
  unaffected, keeping real FK constraints under test.
- `test_thread_meta_workspace_filter.py` covers the three AUTO / explicit
  / None paths.

3 new tests pass; 115 existing thread_meta/run/feedback/run_event/owner
tests stay green.

backend/pyproject.toml

@@ -37,7 +37,8 @@ dev = [
 
 [tool.pytest.ini_options]
 markers = [
-    "no_auto_user: disable the conftest autouse contextvar fixture for this test",
+    "no_auto_user: disable the conftest autouse user contextvar fixture for this test",
+    "no_auto_workspace: disable the conftest autouse workspace contextvar fixture for this test",
     "postgres: requires a Postgres testcontainer (Docker daemon); skipped if unavailable",
 ]