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>

apps/compliance-portal/src/pages/subprocessors/_components/SubprocessorCategorySection.tsx

@@ -38,7 +38,7 @@ export function SubprocessorCategorySection({ category, subprocessors }: Subproc
   return (
     <section className="flex flex-col gap-4">
       <div className="flex flex-col gap-1">
-        <Text size={3} weight="medium" color="neutral" highContrast>
+        <Text size={3} weight="medium" color="neutral" highContrast role="heading" aria-level={2}>
           {t(`categories.${category}.label`)}
         </Text>
         <Text size={2} color="neutral">