Isolate CI/deploy tests from production DATABASE_URL (#24)

## Summary
- Root cause: `docker-compose.yml` used `${DATABASE_URL:-…}`, so the Act runner’s host/`prod` `DATABASE_URL` was interpolated into the web service. Containerized deploy tests then ran against shared Postgres instead of the bundled compose `db`.
- Stop reading host `DATABASE_URL` in compose (use `COMPOSE_DATABASE_URL` override only).
- Deploy docker step: unset DB secrets, dedicated compose project name, `up --wait db`, force test `DATABASE_URL` to `postgres://scha:scha@db:5432/scha`, `down -v` on exit (ephemeral DB).
- Clear `DATABASE_URL`/`DB_HOST` in `unittests.yml` / `ci.yml` so host sqlite-fallback tests cannot hit prod either.

## Test plan
- [ ] Merge to `master` and watch Unit Tests + Deploy docker job.
- [ ] Confirm docker step connects only to compose `db` (no traffic/errors against `10.0.0.230`).
- [ ] Confirm `docker compose -p scha-ci-<sha> …` volumes cleaned after job (`down -v`).
- [ ] Local: `docker compose up --build` still works with bundled Postgres.

Reviewed-on: #24

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