[KW-Search] Store direct parent of every node - Update core api

[tdraier]

In order to get the children of one node we need to store the direct parent of each node in data_sources_nodes table. Although the list of parents is stored and supposedly ordered in parents : ([self, parent, .. root]), this is much more clear and stable to rely on dedicated table. We can still rely on this to backfill data.
Parent field will be optional in db and in the api - only connectors which upserts nodes with hierarchy will have to pass a new parent parameter.

[tdraier]

Note : for the backfill, we need to have #9069 solved so that we store in parent the id in the correct format.

[philipperolet]

Related discussion https://dust4ai.slack.com/archives/C050SM8NSPK/p1733837120557819

[philipperolet]

As discussed, this card is focused on updating core & front API so they get a new parent field ; for the store we will rely on index creation rather