docs: add media-items bulk operation and download documentation

- Add comprehensive documentation for bulk delete endpoint
- Add comprehensive documentation for bulk update endpoint
- Add comprehensive documentation for download endpoint
- Document all request/response fields with correct names
- Include examples and error codes
- Add notes on tag normalization and partial success

docs/developer/api/media-items/download_media_item.md

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+# Download Media Item
+
+Download a media item file (EPUB, PDF, etc.) from the Bookhoard server.
+
+**Endpoint**: `GET /api/media-items/:uuid/download`
+**Auth**: None (public endpoint for Kobo devices)
+**Content-Type**: Binary file download
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| uuid | string | Yes | Media item UUID |
+
+## Response
+
+**Success (200 OK)**: Binary file data
+
+**Response Headers**:
+- `Content-Type`: `application/epub+zip`, `application/pdf`, or appropriate MIME type
+- `Content-Disposition`: `attachment; filename="filename.epub"`
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 404 | Media item not found |
+| 500 | Server error during file download |
+
+## Example
+
+```bash
+curl -O http://localhost:8765/api/media-items/550e8400-e29b-41d4-a716-446655440000/download
+```
+
+## Notes
+
+- **Public endpoint**: No authentication required for Kobo device downloads
+- **File format**: Returns the original file format (EPUB, PDF, etc.)
+- **Kobo integration**: Designed for direct downloads from Kobo e-readers
+- **Cover images**: Use `/api/media-items/:uuid/cover` for cover images