fix(api): pass run_in_background parameter to memify function (#1847)

## Summary

The `run_in_background` parameter was defined in `MemifyPayloadDTO` but
was never passed to the `cognee_memify` function call, making the
parameter effectively unused.

## Changes

This fix passes the `run_in_background` parameter from the payload to
the `cognee_memify` function so users can actually run memify operations
in the background.

## Testing

- `uv run ruff check cognee/api/v1/memify/routers/get_memify_router.py`
- All checks passed
- `uv run ruff format cognee/api/v1/memify/routers/get_memify_router.py`
- No changes needed

## DCO

I affirm that all code in every commit of this pull request conforms to
the terms of the Topoteretes Developer Certificate of Origin

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Fixed background execution flag for memify operations to be properly
applied when requested. The background execution setting is now
correctly propagated through the system, ensuring operations run as
intended.

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

cognee/api/v1/memify/routers/get_memify_router.py

@@ -90,6 +90,7 @@ def get_memify_router() -> APIRouter:
                 dataset=payload.dataset_id if payload.dataset_id else payload.dataset_name,
                 node_name=payload.node_name,
                 user=user,
+                run_in_background=payload.run_in_background,
             )
 
             if isinstance(memify_run, PipelineRunErrored):