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/integration/minio/conftest.py

@@ -1,39 +0,0 @@
-from collections.abc import Iterator
-
-import pytest
-from testcontainers.community.minio import MinioContainer
-
-from src.config import MinioSettings
-from src.infrastructure.minio.client import create_client
-
-_BUCKET = "test-source-files"
-
-
-@pytest.fixture(scope="session")
-def minio_container() -> Iterator[MinioContainer]:
-    with MinioContainer() as container:
-        yield container
-
-
-@pytest.fixture(scope="session")
-def minio_settings(minio_container: MinioContainer) -> MinioSettings:
-    config = minio_container.get_config()
-    # Pinned to IPv4 for the same reason as postgres_url (see
-    # tests/integration/postgres/conftest.py): `localhost` resolves to `::1`
-    # first, but Docker only publishes the mapped port on IPv4, so the
-    # connection hangs instead of failing.
-    endpoint = config["endpoint"].replace("localhost:", "127.0.0.1:")
-    return MinioSettings(
-        endpoint=endpoint,
-        access_key=config["access_key"],
-        secret_key=config["secret_key"],
-        secure=False,
-        bucket=_BUCKET,
-    )
-
-
-@pytest.fixture(scope="session", autouse=True)
-def _ensure_bucket(minio_settings: MinioSettings) -> None:
-    client = create_client(minio_settings)
-    if not client.bucket_exists(minio_settings.bucket):
-        client.make_bucket(minio_settings.bucket)