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/set_library_visibility.md

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+# Set Library Visibility
+
+Set library visibility for a specific user (Admin only).
+
+**Endpoint**: `POST /api/libraries/visibility`
+**Auth**: Required (Admin only)
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| user_id | string | Yes | User UUID |
+| library_id | string | Yes | Library UUID |
+| is_visible | boolean | Yes | Whether library is visible to user |
+
+### Example Request
+
+```json
+{
+  "user_id": "user-uuid",
+  "library_id": "library-uuid",
+  "is_visible": true
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "user_id": "user-uuid",
+  "library_id": "library-uuid",
+  "is_visible": true
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid input data |
+| 401 | Invalid or expired token |
+| 403 | User is not an admin |
+| 404 | User or library not found |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->