refactor(test): share container fixtures across the integration and e2e suites

Why:
- tests/e2e/ cannot reach fixtures defined in a per-boundary conftest, and
  `pytest_plugins` is only honoured in the root conftest.

Changes:
- move the Postgres/MinIO/Qdrant container fixtures into
  tests/support/containers.py and register it as a root plugin
- fold MinIO bucket creation into `minio_settings`; an autouse fixture in a
  globally registered plugin would pull a container into unit runs
- add a `postgres_settings` fixture so a component can be built from it directly

Impact:
- no behavior change; `pytest -m unit` still needs no Docker

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/conftest.py

@@ -10,6 +10,12 @@ from httpx import ASGITransport, AsyncClient
 from src.config import Settings
 from src.main import create_app
 
+# Disposable Postgres/MinIO/Qdrant containers (ADR-0016). Registered here
+# because `pytest_plugins` is only honoured in the root conftest, and both
+# `tests/integration/*/` and `tests/e2e/` need the same containers. The
+# fixtures are session-scoped but lazy, so unit runs still need no Docker.
+pytest_plugins = ["tests.support.containers"]
+
 
 @pytest.fixture(autouse=True)
 def _reset_structlog_after_test() -> Iterator[None]: