Add BulkCreator for batch insert operations in repository base

## Motivation

Currently, `execute_creator` handles single row inserts. When inserting multiple rows,
calling it N times results in N flush operations, which is inefficient.

## Implementation

Add `BulkCreator` and `execute_bulk_creator` to handle multiple row inserts with:

- Single flush operation for all rows
- Consistent pattern with existing Creator abstraction

## Scope

- Add `BulkCreator` dataclass
- Add `BulkCreatorResult` dataclass
- Add `execute_bulk_creator` function
- Location: `src/ai/backend/manager/repositories/base/creator.py`

JIRA Issue: BA-3774