feat: add CategoryImportController for bulk JSON import and export of categories

- Implemented export functionality to retrieve all rows from specified category tables.
- Developed import functionality with strict validation and referential integrity checks.
- Added error handling for various import scenarios including invalid formats and duplicate entries.
- Introduced tests for import functionality to ensure correct behavior and validation.

docs/api/tag.md

@@ -130,3 +130,10 @@ Delete a tag.
 | `ERR_AUTH_001` | 401 | Missing token |
 | `ERR_AUTH_006` | 403 | Not admin |
 | `ERR_NOT_FOUND_001` | 404 | Tag not found |
+
+---
+
+## Bulk import / export
+
+Full-table JSON export and strict wipe+replace import for this category live under
+`/api/v1/admin/categories/{bundle}/{export|import}` — see [category-import.md](category-import.md).