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/create_library.md

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+# Create Library
+
+Create a new library (Admin only).
+
+**Endpoint**: `POST /api/libraries`
+**Auth**: Required (Admin only)
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| name | string | Yes | Library name |
+| description | string | No | Library description |
+| type | string | Yes | Library type (e.g., "ebooks", "comics", "audiobooks") |
+
+### Example Request
+
+```json
+{
+  "name": "Comics Collection",
+  "description": "Digital comics",
+  "type": "comics"
+}
+```
+
+## Response (201 Created)
+
+```json
+{
+  "id": "uuid",
+  "name": "Comics Collection",
+  "description": "Digital comics",
+  "type": "comics",
+  "created_at": "2026-01-31T10:00:00Z"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid input data |
+| 401 | Invalid or expired token |
+| 403 | User is not an admin |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->