✨ feat(cache): 幂等只读接口接入缓存

- 新增 withCache 包装：命中直接返回，miss 回源后写入，仅缓存成功响应
- 详情/目录/批量详情/章节摘要缓存 24 小时；搜索/分类列表/分类详情短缓存 10 分钟
- 7 个 fqapi 处理函数原函数体不动、改名 fetchXxx，外层套缓存壳
- redisGetCache 补 3 秒读超时，与写路径一致，防远程 Redis 网络抖动拖垮请求
- 新增 TestWithCache：验证命中不回源、失败响应不缓存

internal/fqapi/cache_manager_test.go

@@ -55,3 +55,46 @@ func TestConcurrentCacheAccess(t *testing.T) {
 	}
 	wg.Wait()
 }
+
+// TestWithCache 验证缓存命中不回源、失败响应不缓存
+func TestWithCache(t *testing.T) {
+	if err := config.LoadConfig("../../config/config.yaml"); err != nil {
+		t.Skipf("无法加载配置: %v", err)
+	}
+	db, err := sql.Open("sqlite3", t.TempDir()+"/test.db?_journal_mode=WAL&_busy_timeout=5000")
+	if err != nil {
+		t.Fatal(err)
+	}
+	oldDB, oldDriver := cacheDB, dbDriver
+	cacheDB, dbDriver = db, "sqlite3"
+	defer func() {
+		cacheDB, dbDriver = oldDB, oldDriver
+		db.Close()
+	}()
+	if _, err := db.Exec(`CREATE TABLE IF NOT EXISTS cache (
+		key TEXT PRIMARY KEY, value TEXT,
+		created_at INTEGER DEFAULT (strftime('%s', 'now'))
+	)`); err != nil {
+		t.Fatal(err)
+	}
+
+	calls := 0
+	fetch := func() (string, error) { calls++; return "result", nil }
+	if got, _ := withCache("wc_hit", 60, fetch); got != "result" {
+		t.Fatalf("首次应返回回源结果, got %q", got)
+	}
+	if got, _ := withCache("wc_hit", 60, fetch); got != "result" {
+		t.Fatalf("命中应返回缓存结果, got %q", got)
+	}
+	if calls != 1 {
+		t.Fatalf("命中缓存不应回源, fetch 调用 %d 次", calls)
+	}
+
+	errCalls := 0
+	errFetch := func() (string, error) { errCalls++; return "", fmt.Errorf("boom") }
+	withCache("wc_miss", 60, errFetch)
+	withCache("wc_miss", 60, errFetch)
+	if errCalls != 2 {
+		t.Fatalf("失败响应不应被缓存, fetch 调用 %d 次", errCalls)
+	}
+}