Read-only SPCS writeback-readiness signal (no side effects).

Snowflake mode only (DVT-2905 r3 review fix, HIGH-1). The READ-ONLY companion to POST /v1/connections/host/test's writeback field: that route runs four caller's-rights checks AND mutates shared state on every call (stamps lastTestedAt/lastTestError onto the org-shared host connection row, writes a connection.test audit row, and can flip /admin/health's needs-attention surface) — a page-mount effect that only wants the writeback-readiness signal must not pay that side effect just to render.

This route calls the SAME engine-side readiness plumbing (POST /internal/writeback/readiness) but skips the four caller's-rights checks entirely and never touches RecordConnectionTest or the audit log. It always requests the engine's cached-first behavior (fresh: false) — honoring the engine's per-process provisioned-cache memo rather than paying for a live Snowflake connection on every page load; the explicit "Verify setup" button in the admin panel still goes through POST /v1/connections/host/test (fresh: true) for a live diagnostic.

Any authenticated member holding data:query may call it — same scope gate as POST /v1/connections/host/test. Off snowflake mode, this route does not exist (404, leak-safe).