[TASK] Implement clean assets endpoints

Description

Implement POST /v1/maintenance/_cleanAssets and GET /v1/maintenance/_cleanAssets in MaintenanceResource.java. POST starts a background thread that walks the assets directory and deletes orphan binary folders with no matching contentlet. GET returns the current progress status.

Acceptance Criteria

POST /v1/maintenance/_cleanAssets

• Endpoint must start CleanAssetsThread via CleanAssetsThread.getInstance(true, true)
• API caller must receive the initial process status (totalFiles, currentFiles, deleted, running, status)
• Must not start a new thread if one is already running
• Must require CMS Administrator role and Maintenance portlet access

GET /v1/maintenance/_cleanAssets

• API caller must receive current process status: totalFiles, currentFiles, deleted, running (boolean), status (string)
• Status values: "Starting", "Counting", "Cleaning", "Finished", or "Error: ..."
• Must require CMS Administrator role and Maintenance portlet access

Priority

None

Additional Context

• CleanAssetsThread is a singleton thread that walks assets/0-f/0-f/* hex directory structure
• For each directory: looks up contentlet by inode via ContentletAPI.find(inode), deletes if not found or has no identifier
• Progress is updated via BasicProcessStatus and read via buildStatusMap() (uses BeanUtils.describe())
• Angular should poll GET at ~1 second interval while running === "true"
• DWR legacy: CMSMaintenanceAjax.cleanAssets() and getCleanAssetsStatus()

Start in MaintenanceResource.java and read CleanAssetsThread.getInstance(true, true), BasicProcessStatus, and buildStatusMap(). Trace the legacy CMSMaintenanceAjax.cleanAssets() and getCleanAssetsStatus() entry points for expected behavior. Done means POST and GET expose the specified progress fields, avoid duplicate runs, enforce the CMS Administrator and Maintenance portlet requirements, and return the documented status values.