test: standalone test container, isolated from the live prod stack

Adds a `test` stage to the Dockerfile (prod deps + pytest + aiosqlite via
the `dev` extras, never shipped) and a docker-compose.test.yml that runs
it under its own Compose project name (`cassandra-test`). The project-name
isolation matters because this host runs prod — a wrong `compose up` would
otherwise recreate the live `app` container; namespaced project means the
test container can't touch any prod container/network/volume.

Tests use an in-memory aiosqlite DB (per tests/conftest.py) so the
container has no MariaDB / Redis dependency and nothing on the prod DB
is observed or mutated.

Also adds aiosqlite to dev extras — tests have always implicitly needed
it (the conftest pins DATABASE_URL to sqlite+aiosqlite:///:memory:); the
declaration was just missing.

Usage:
  docker compose -f docker-compose.test.yml run --rm test
  docker compose -f docker-compose.test.yml run --rm test pytest -k unsubscribe

pyproject.toml

@@ -30,6 +30,7 @@ dev = [
     "pytest>=8.3",
     "pytest-asyncio>=0.24",
     "pytest-httpx>=0.34",
+    "aiosqlite>=0.20",
     "ruff>=0.7",
 ]