Harden subprocessor filters per review feedback

Address the review comments on the subprocessors work:

- Reject invalid category/country filter values in the Subprocessors
  resolver with an INVALID error instead of relying solely on transport
  coercion, so a malformed request fails fast and explicitly.
- Use pgx.StrictNamedArgs in the new distinct facet queries so missing
  or extra SQL placeholders stay detectable, matching sibling queries.
- Default a nil ThirdPartyFilter at the service boundary to avoid a nil
  dereference in the coredata list/count paths.
- Expose the category group label as an aria heading for assistive tech.
- Add the missing space in the Select "Selected:" story label.

Signed-off-by: Émile Ré <emile@probo.com>

packages/ui/src/v2/Select/Select.stories.tsx

@@ -87,6 +87,7 @@ export function Controlled() {
       </div>
       <span className="text-2 text-sand-11">
         Selected:
+        {" "}
         {value ?? "none"}
       </span>
     </div>