fix(confidence-engine): log case-start failures

app/api/cases/start/route.js

@@ -16,10 +16,25 @@ export async function POST(request) {
           ? result.statusCode
           : 500;
 
+    const diagnostics = {
+      status,
+      error: result.error ?? "Start case failed",
+      validationErrors: result.validationErrors,
+      analysisErrors: result.analysisErrors,
+      validationIssues: result.validationIssues,
+      rawResponse: result.rawResponse ?? undefined,
+    };
+
+    if (status >= 500) {
+      console.error("[api/cases/start] error response", diagnostics);
+    } else {
+      console.warn("[api/cases/start] error response", diagnostics);
+    }
+
     return Response.json(
       {
         success: false,
-        error: result.error ?? "Start case failed",
+        error: diagnostics.error,
         validationErrors: result.validationErrors,
         diagnostics: result.diagnostics,
         analysisErrors: result.analysisErrors,
@@ -28,7 +43,12 @@ export async function POST(request) {
       },
       { status },
     );
-  } catch {
+  } catch (error) {
+    console.error("[api/cases/start] unhandled exception", {
+      message: error instanceof Error ? error.message : String(error),
+      stack: error instanceof Error ? error.stack : undefined,
+      error,
+    });
     return Response.json(
       {
         success: false,