feat(hosted-file): implement intelligent ID resolution for V3 download action

- Updates download_file_action to automatically resolve container IDs (like event_file) to the underlying hosted_file.
- Updates GUIDE__V3_FRONTEND_API.md to document the 'ID Vision' standard for downloads.
- Resolves 404 errors observed when frontend passed event_file IDs to the hosted_file download endpoint.

documentation/GUIDE__V3_FRONTEND_API.md

@@ -89,6 +89,7 @@ V3 uses specialized **"Action"** routes for binary operations to separate proces
 **Path**: `GET /v3/action/hosted_file/{id}/download`
 
 **Features:**
+- **ID Vision:** Automatically resolves `{id}` if it belongs to a container object (e.g., `event_file`) instead of a direct `hosted_file`.
 - **Streaming:** Supports standard `Range` headers for large files and video seeking.
 - **Auth Bypass:** Use `?site_key=<auth_key>` to download without an API Key header or JWT (useful for public kiosks).
 - **Testing:** Supports `delay_ms` query parameter.