Isolate CI/deploy tests from host/prod DATABASE_URL.

Compose was interpolating ${DATABASE_URL} from the Act runner, so containerized tests could hit shared Postgres. Use COMPOSE_DATABASE_URL, an ephemeral compose project with down -v, and clear DB env in unit-test workflows.

docker-compose.yml

@@ -20,12 +20,16 @@ services:
       - "8000:8000"
     # No required env_file — CI has no .env. Defaults below; for local secrets:
     #   docker compose --env-file .env up
+    #
+    # Do NOT interpolate ${DATABASE_URL} here. On the Act runner / control node that
+    # var often points at shared prod/beta Postgres; compose would bake it into
+    # containerized tests. Use COMPOSE_DATABASE_URL only if you need to override.
     environment:
       DJANGO_ENV: ${DJANGO_ENV:-dev}
       DJANGO_SECRET_KEY: ${DJANGO_SECRET_KEY:-dev-only-change-me}
       DJANGO_DEBUG: ${DJANGO_DEBUG:-true}
       DJANGO_ALLOWED_HOSTS: ${DJANGO_ALLOWED_HOSTS:-localhost,127.0.0.1,0.0.0.0}
-      DATABASE_URL: ${DATABASE_URL:-postgres://scha:scha@db:5432/scha}
+      DATABASE_URL: ${COMPOSE_DATABASE_URL:-postgres://scha:scha@db:5432/scha}
     depends_on:
       db:
         condition: service_healthy