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

.gitea/workflows/unittests.yml

@@ -25,5 +25,8 @@ jobs:
         env:
           DJANGO_ENV: dev
           DJANGO_SECRET_KEY: test-secret-key
+          # Explicitly clear DB vars so host/prod DATABASE_URL cannot leak in.
+          DATABASE_URL: ""
+          DB_HOST: ""
         run: |
           uv run python manage.py test