fix: 安全性与代码质量加固（异常保护、外键级联、竞态修复、优雅关机）

src/server/app.ts

@@ -218,7 +218,12 @@ function createTargetsResponse(store: ProbeStore): TargetStatus[] {
 function mapCheckResult(row: StoredCheckResult): CheckResult {
   let failure: CheckFailure | null = null;
   if (row.failure) {
-    failure = JSON.parse(row.failure) as CheckFailure;
+    try {
+      failure = JSON.parse(row.failure) as CheckFailure;
+    } catch {
+      console.warn(`无法解析 failure 数据: target_id=${row.target_id}, timestamp=${row.timestamp}`);
+      failure = null;
+    }
   }
 
   return {