[KW-Search] Backfill `nodes` table with documents

[philipperolet]

Modify existing document endpoints to write to both documents and nodes tables

Related design doc section:
Changes in documents endpoint

Todo:
Update POST endpoint to write to both tables
Update DELETE to handle both tables
Update parents endpoint to also update the nodes table
Important points:
At this time, parents are written in nodes but not read from it
Handle transactions properly across both tables
Depends on task(s):
Create new core tables and basic table ops