Post Signup Pending

Create or resume a pending signup. Returns the User's id as pendingId.

Maps service-layer ValueErrors to 400 with the error code in the body so the frontend can show a localized message based on the code, not the English error string. Rate-limited to 20 req/min/IP via slowapi — sufficient for legitimate retries while throttling enumeration attempts.

Returns 201 Created for fresh inserts; 200 OK when a live-pending or abandoned row is resumed (the row already existed).