refactor: 统一 target name/description 可空语义，前端展示 fallback 到 id

- schema: name/description 允许省略或显式 null，TypeBox Union([Null, String])
- 类型: RawTargetConfig/ResolvedTargetBase/子类型/StoredTarget/TargetStatus name 改为 string | null
- checker resolve: name: t.name ?? null，不再 fallback 到 id
- 语义校验: 拒绝空字符串和纯空白 name
- SQLite: targets.name 列改为可空 TEXT
- 前端: 新增 getTargetDisplayName(target) 展示 name ?? id
- 测试: 覆盖 name/description null 全场景，查找改为按 id
- 文档: 更新 README/DEVELOPMENT 和 6 个 openspec specs

src/server/checker/runner/db/execute.ts

@@ -185,7 +185,7 @@ export class DbChecker implements CheckerDefinition<ResolvedDbTarget> {
       group: target.group ?? "default",
       id: t.id,
       intervalMs: context.defaultIntervalMs,
-      name: t.name ?? t.id,
+      name: t.name ?? null,
       timeoutMs: context.defaultTimeoutMs,
       type: "db",
     } satisfies ResolvedDbTarget;

src/server/checker/runner/db/types.ts

@@ -21,7 +21,7 @@ export interface ResolvedDbTarget extends ResolvedTargetBase {
   expect?: DbExpectConfig;
   group: string;
   intervalMs: number;
-  name: string;
+  name: null | string;
   timeoutMs: number;
   type: "db";
 }