docs(api): add universal progress tracking documentation

- get_universal_progress.md - GET /api/progress/:id
- update_universal_progress.md - POST /api/progress/:id
- get_progress_history.md - GET /api/progress/:id/history

Documents device-agnostic (universal) reading progress tracking
that works across all devices (Kobo, KOReader, etc.)

docs/developer/api/libraries/delete_library_folder.md

@@ -0,0 +1,44 @@
+# Delete Library Folder
+
+Delete a folder from a library.
+
+**Endpoint**: `DELETE /api/libraries/:id/folders`
+**Auth**: Required (Admin only)
+**Content-Type**: `application/json`
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| id | string (UUID) | Yes | Library UUID |
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| folder_path | string | Yes | Absolute path to the folder to delete |
+
+### Example Request
+
+```json
+{
+  "folder_path": "/path/to/library/folder"
+}
+```
+
+## Response (204 No Content)
+
+Library folder deleted successfully.
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid request data |
+| 401 | Invalid or expired token |
+| 403 | User does not have admin privileges |
+| 404 | Library or folder not found |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->