The @config-editing tests need the gateway booted from the config-in-database
read path (TENSORZERO_INTERNAL_FLAG_ENABLE_CONFIG_IN_DATABASE=true) with a
pre-loaded config — they cannot run in the standard ui-tests-e2e-main job.

Additionally, the tests make direct API calls to $TENSORZERO_GATEWAY_URL from
the Playwright runner (host process), but the main e2e job sets that variable
to http://gateway:3000 which is only resolvable within the Docker network,
causing EAI_AGAIN failures.

Changes:
- Tag all tests in config_editing.spec.ts with @config-editing so they can be
  selected or excluded via --grep / --grep-invert.
- Add @config-editing to the --grep-invert filter in ui-tests-e2e-main for all
  three auth_mode matrix legs.
- Add ui/fixtures/docker-compose.config-in-db.yml: a Docker Compose override
  that adds a gateway-store-config one-shot service (stores the UI fixtures
  config into Postgres) and rewires the gateway to load from the database.
- Add ui-tests-e2e-config-editing CI job: starts the stack with the
  config-in-db override, then runs playwright with --grep @config-editing and
  TENSORZERO_GATEWAY_URL=http://localhost:3000 so the host-side runner can
  reach the gateway via the mapped port.