DB effect facade + backend behaviour.
Nodes call Bloccs.Effects.DB.insert/3 against ctx.effects.db; the facade
dispatches to the bound backend (DB.Mock in tests, DB.Ecto against a real
repo in production). Scope enforcement ("table:action") is the backend's
job. Backends @behaviour Bloccs.Effects.DB.
Summary
Functions
Insert attrs into table through the bound DB backend.