perf(storage): add config caching and shared HTTP connection pool

- Implement thread-safe local config caching with a 5s TTL check.
- Reuse backend client singletons in storage.Active and storage.ForDriver.
- Reset in-memory config cache on configuration saves and updates.
- Create internal/httppool package to manage shared HTTP transports.
- Configure WebDAV client and CDN retrieval to reuse the shared pool.
- Add unit tests for httppool and storage caching behaviors.

internal/apps/admin/system_config/routers.go

@@ -246,6 +246,10 @@ func UpdateSystemConfig(c *gin.Context) {
 		return
 	}
 
+	if key == model.ConfigKeyStorageConfig {
+		storage.ResetCache()
+	}
+
 	c.JSON(http.StatusOK, util.OKNil())
 }