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

desktop/src-tauri/src/classroom_commands/mod.rs

@@ -55,7 +55,9 @@ pub async fn init_persistence(
         .map_err(|e| format!("Failed to get app data dir: {}", e))?;
 
     let db_path = app_dir.join("classroom").join("classrooms.db");
-    std::fs::create_dir_all(db_path.parent().unwrap())
+    let db_dir = db_path.parent()
+        .ok_or_else(|| "Invalid classroom database path: no parent directory".to_string())?;
+    std::fs::create_dir_all(db_dir)
         .map_err(|e| format!("Failed to create classroom dir: {}", e))?;
 
     let persistence: ClassroomPersistence = ClassroomPersistence::open(db_path).await?;