Fix nested try-catch structure in checkout route

Add missing catch and closing brace for outer try block.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/app/api/checkout/route.ts

@@ -153,4 +153,8 @@ export async function POST(req: Request) {
     console.error('Returning error to client:', message);
     return NextResponse.json({ error: message }, { status: 500 });
   }
+  } catch (err) {
+    const message = err instanceof Error ? err.message : 'Something went wrong.';
+    return NextResponse.json({ error: message }, { status: 500 });
+  }
 }