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/media-items/search_media_items.md

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+# Search Media Items
+
+Search for media items by title, author, or description.
+
+**Endpoint**: `GET /api/media-items/search`
+**Auth**: Required
+
+## Query Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| q | string | Yes | Search query (minimum 2 characters) |
+| limit | integer | No | Number of results (default 20) |
+| offset | integer | No | Number to skip |
+
+## Request Headers
+
+| Header | Type | Required | Description |
+|--------|------|-----------|-------------|
+| Authorization | string | Yes | Bearer token |
+
+### Example Request
+
+```http
+GET /api/media-items/search?q=Harry+Potter&limit=20&offset=0
+Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
+```
+
+## Response (200 OK)
+
+```json
+{
+  "results": [
+    {
+      "id": "uuid",
+      "title": "Book Title",
+      "author": "Author Name",
+      "match_score": 0.95
+    }
+  ],
+  "total": 15
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid search query (too short) |
+| 401 | Invalid or expired token |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->