chore(workbench): log the structure failure reason and source segment count

Cloudflare replaces an origin 502 body with its own error page, so the JSON
message never reached the client. Log the reason server-side (codes/messages
only, never source text).

app/api/0pt/documents/structure/route.ts

@@ -244,6 +244,7 @@ export async function POST(request: NextRequest) {
     );
   }
 
+  console.log('[0pt-workbench] doc0 source read:', sources.sources.length, 'segments');
   const sourceChars = sources.sources.reduce((total, source) => total + source.text.length, 0);
   const estimatedInputTokens = Math.ceil(JSON.stringify(sources.sources).length / 4) + 1024;
   if (estimatedInputTokens > policy.maxInputTokens) {
@@ -433,6 +434,7 @@ export async function POST(request: NextRequest) {
       }
     }
     const message = error instanceof Error ? error.message : 'Structuring failed';
+    console.error('[0pt-workbench] structure failed:', message);
     return NextResponse.json(
       { error: 'STRUCTURE_FAILED', message, reservation_id: reservationId },
       { status: 502 },