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/checker/store.ts

@@ -26,7 +26,7 @@ CREATE TABLE IF NOT EXISTS check_results (
   duration_ms REAL,
   status_detail TEXT,
   failure TEXT,
-  FOREIGN KEY (target_id) REFERENCES targets(id)
+  FOREIGN KEY (target_id) REFERENCES targets(id) ON DELETE CASCADE
 )
 `;
 
@@ -43,6 +43,7 @@ export class ProbeStore {
     ensureDir(dirname(dbPath));
     this.db = new Database(dbPath, { create: true });
     this.db.run("PRAGMA journal_mode = WAL");
+    this.db.run("PRAGMA foreign_keys = ON");
     this.db.run(CREATE_TARGETS_TABLE);
     this.db.run(CREATE_CHECK_RESULTS_TABLE);
     this.db.run(CREATE_INDEX);