Fix VIES check treating "member state unavailable" as "invalid"

VIES answers HTTP 200 even when it couldn't actually perform the check
(actionSucceed: false, e.g. MS_UNAVAILABLE — Germany's own national
gateway does this fairly regularly). checkVatIdViaVies() only ever read
data.valid, which is absent on that response shape, so it silently
read as valid: false — a real, currently-registered German VAT ID
(reported: DE351362947) looked rejected. Worse, /api/checkout/validate-
vat then wrapped even a correctly-returned ok:false as { ok: true,
valid: false }, which the client reads as "invalid" rather than
"unavailable" — the actual bug the user hit, compounding the vies.ts
gap. Both are fixed now: an unconfirmable check surfaces to the client
as ok:false, which CheckoutContent.tsx's handleVatIdBlur already
correctly renders as "USt-IdNr.-Prüfung derzeit nicht möglich"
instead of a rejection. Same fix applied to the payload backend's own
copy of vies.ts (company-settings' VAT check).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/api/checkout/validate-vat/route.ts

@@ -24,7 +24,16 @@ export async function POST(request: Request) {
 
   const result = await checkVatIdViaVies(normalized);
   if (!result.ok) {
-    return NextResponse.json({ ok: true, valid: false, reason: `USt-IdNr.-Prüfung derzeit nicht möglich (${result.reason}).` });
+    // `ok: false` here means "VIES couldn't confirm this one way or the
+    // other" (unreachable, or the member state's own gateway is briefly
+    // down — `MS_UNAVAILABLE`, which VIES itself answers 200 for, not an
+    // error status) — NOT "confirmed invalid". Previously this branch
+    // still answered `{ ok: true, valid: false }`, which the client reads
+    // as a rejected VAT ID (`vatIdViesStatus = "invalid"`) instead of
+    // "couldn't check right now" (`"unavailable"`) — a real, currently
+    // registered VAT ID looked wrong to the customer whenever VIES (or
+    // just Germany's own national gateway) had a hiccup.
+    return NextResponse.json({ ok: false, reason: result.reason });
   }
   return NextResponse.json({ ok: true, valid: result.valid, name: result.name });
 }