feat(api): add hidden folders management endpoints to v2 API

Implements three new endpoints for managing repository hidden folders:
- GET /repos/{owner}/{repo}/hidden-folders - list all hidden folders
- PUT /repos/{owner}/{repo}/hidden-folders - add a hidden folder
- DELETE /repos/{owner}/{repo}/hidden-folders - remove a hidden folder

All write operations require repository admin permissions.

modules/structs/repo_hidden_folder.go

@@ -0,0 +1,20 @@
+// Copyright 2026 MarketAlly. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package structs
+
+// HiddenFolderV2 represents a hidden folder in a repository.
+type HiddenFolderV2 struct {
+	Path      string `json:"path"`
+	CreatedAt int64  `json:"created_at"`
+}
+
+// HiddenFolderListV2 is the response for listing hidden folders.
+type HiddenFolderListV2 struct {
+	Folders []*HiddenFolderV2 `json:"folders"`
+}
+
+// HiddenFolderOptionV2 is the request body for adding or removing a hidden folder.
+type HiddenFolderOptionV2 struct {
+	Path string `json:"path" binding:"Required"`
+}