fix: 发布前审计 Batch 2 — Debug遮蔽 + unwrap + 静默吞错 + MCP锁 + 索引 + Config验证

安全:
- LlmConfig 自定义 Debug impl，api_key 显示为 "***REDACTED***"
- tsconfig.json 移除 ErrorBoundary.tsx 排除项（安全关键组件）
- billing/handlers.rs Response builder unwrap → map_err 错误传播
- classroom_commands/mod.rs db_path.parent().unwrap() → ok_or_else

静默吞错:
- approvals.rs 3处 warn→error（审批状态丢失是严重事件）
- events.rs publish() 添加 Event dropped debug 日志
- mcp_transport.rs eprintln→tracing::warn (僵尸进程风险)
- zclaw-growth sqlite.rs 4处迁移：区分 duplicate column name 与真实错误

MCP Transport:
- 合并 stdin+stdout 为单一 Mutex<TransportHandles>
- send_request write-then-read 原子化，防止并发响应错配

数据库:
- 新迁移 20260418000001: idx_rle_created_at + idx_billing_sub_plan + idx_ki_created_by

配置验证:
- SaaSConfig::load() 添加 jwt_expiration_hours>=1, max_connections>0, min<=max

crates/zclaw-saas/src/billing/handlers.rs

@@ -587,7 +587,7 @@ pub async fn get_invoice_pdf(
         })?;
 
     // 返回 PDF 响应
-    Ok(axum::response::Response::builder()
+    axum::response::Response::builder()
         .status(200)
         .header("Content-Type", "application/pdf")
         .header(
@@ -595,5 +595,8 @@ pub async fn get_invoice_pdf(
             format!("attachment; filename=\"invoice-{}.pdf\"", invoice.id),
         )
         .body(axum::body::Body::from(bytes))
-        .unwrap())
+        .map_err(|e| {
+            tracing::error!("Failed to build PDF response: {}", e);
+            SaasError::Internal("PDF 响应构建失败".into())
+        })
 }