Shared definition for the polymorphic mnemosyne_nodes table schema.
All node types (semantic, episodic, procedural, etc.) live in a single table
with JSON data/links fields and a vector embedding column. The embedding
column type is the only thing that differs between database engines, so the
concrete schema modules (MnemosyneEcto.Schema.Node.Postgres and
MnemosyneEcto.Schema.Node.SQLite) inject it via use.
use MnemosyneEcto.Schema.Node, embedding_type: Pgvector.Ecto.Vector