core (memory/sqla_mapping): fix some technical debt: a first Alembic revision file is joined to migrate existing databases to new mapping. This patch: - set `nullable=False` in various places - drop legacy `message_types` table, and use `Enum` instead - use naming convention, and get rid of anonymous constraint (this is the main reason why the revision file is so long)

version: "3.6"
services:

  backend:
    image: libervia/backend:dev
    build: backend-dev

  web:
    image: libervia/web:dev
    build: libervia-web-dev
    depends_on:
      - backend