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

README.md

@@ -27,7 +27,8 @@ Without `DATABASE_URL` / `DB_HOST`, settings fall back to SQLite (`db.sqlite3`).
 docker compose up --build
 ```
 
-App: http://localhost:8000 — Postgres via `DATABASE_URL=postgres://scha:scha@db:5432/scha`.
+App: http://localhost:8000 — Postgres via bundled `db` (`postgres://scha:scha@db:5432/scha`).
+Compose does **not** read host `DATABASE_URL` (avoids CI/prod leaks); override with `COMPOSE_DATABASE_URL` if needed.
 
 ## Environment variables
 
@@ -69,7 +70,7 @@ Validate with:
 |----------|---------|--------|
 | `unittests.yml` | push + PR → `master` | `uv sync` + `manage.py test` |
 | `ci.yml` | PR → `master` | same unit tests |
-| `deploy.yml` | after Unit Tests succeeds on `master` **push** | docker build/test → `deploy.sh` |
+| `deploy.yml` | after Unit Tests succeeds on `master` **push** | docker build + tests on **ephemeral compose Postgres** → `deploy.sh` |
 
 Deploy never runs on PRs.