feat: 新增 DB checker — 支持 PostgreSQL/MySQL/SQLite 连接测试与 SQL 查询断言

- 实现 db 类型 checker，使用 Bun 内置 SQL 类
- 支持 db.url 连接字符串和可选 db.query 查询语句
- expect 支持 maxDurationMs、rowCount、rows 逐列校验
- 凭据屏蔽序列化输出
- SQLite 内存数据库测试覆盖

probes.example.yaml

@@ -232,3 +232,39 @@ targets:
       exitCode: [1]
       stderr:
         - contains: "simulated error"
+
+  # ========== DB targets ==========
+
+  - name: "SQLite 内存数据库连接测试"
+    type: db
+    group: "数据库"
+    db:
+      url: "sqlite://:memory:"
+    expect:
+      maxDurationMs: 1000
+
+  - name: "SQLite 内存数据库查询行数"
+    type: db
+    db:
+      url: "sqlite://:memory:"
+      query: "SELECT 1 as cnt"
+    expect:
+      maxDurationMs: 1000
+      rowCount:
+        gte: 1
+
+  - name: "SQLite 内存数据库多列结果校验"
+    type: db
+    db:
+      url: "sqlite://:memory:"
+      query: "SELECT 1 as id, 'Alice' as name, 'engineer' as role"
+    expect:
+      rowCount:
+        equals: 1
+      rows:
+        - id:
+            gte: 1
+          name:
+            exists: true
+          role:
+            contains: "engineer"