docs: add library and media items API endpoints

Phase 2 part 3: Split library and media item endpoints
- Libraries: get_visible_libraries, get_library, create_library, add_library_folder, set_library_visibility
- Media Items: list_media_items, get_media_item, search_media_items, filter_sort_media_items, update_media_item, delete_media_item
- Complete request/response examples for all endpoints

docs/api/libraries/get_library.md

@@ -0,0 +1,55 @@
+# Get Library Details
+
+Retrieve details of a specific library.
+
+**Endpoint**: `GET /api/libraries/{library_id}`
+**Auth**: Required
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| library_id | string | Yes | Library UUID |
+
+## Request Headers
+
+| Header | Type | Required | Description |
+|--------|------|-----------|-------------|
+| Authorization | string | Yes | Bearer token |
+
+### Example Request
+
+```http
+GET /api/libraries/uuid-here
+Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
+```
+
+## Response (200 OK)
+
+```json
+{
+  "id": "uuid",
+  "name": "My Ebooks",
+  "description": "Ebook collection",
+  "type": "ebooks",
+  "folders": [
+    {
+      "id": "uuid",
+      "folder_path": "/path/to/ebooks"
+    }
+  ],
+  "is_visible": true
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Invalid or expired token |
+| 403 | User does not have access to this library |
+| 404 | Library not found |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->