`ThreadMetaStore.check_access(thread_id, user_id, workspace_id, *,
require_existing)` is now a three-positional method. Cross-workspace is
denied unconditionally — even when the row exists and `user_id` matches
— so the decorator layer can convert the False into a **404** and never
leak the existence of a thread across tenants. Inside the workspace, the
existing legacy semantics still hold (NULL `row.user_id` stays
"shared in workspace", `require_existing` still gates the missing-row
path against ghost-row re-targeting).
`@require_permission(owner_check=True)` in `app/gateway/authz.py` now
reads the active workspace from `get_effective_workspace_id()` (set by
PR4 AuthMiddleware; falls back to "default" in no-auth dev) and passes
it through. The existing 404-not-403 mapping is unchanged.
Existing positional callers in `test_thread_meta_repo.py` and the
permissive mock in `test_threads_router.py` were updated for the new
arity. 58 thread_meta / router / memory tests stay green; 90 auth /
uploads / suggestions tests stay green.
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