Run the indexer as a background job when running in local Cognita mode

[innoavator]

Currently when you create a collection locally, indexing is triggered automatically by frontend and it runs in the same process. As a result, collection creation blocks all other API calls
Hence we would want indexing to start as a separate background process that does not block the collection creation.

Starting point is:

• ingest api in backend/server/routers/collection.py and trace back the necessary files from there.