test(postgres): allow Testcontainers to override the migration sqlalchemy.url

alembic/env.py previously always overwrote sqlalchemy.url from
Settings().postgres.dsn, which made it impossible for a test fixture to
point Alembic at a Testcontainers-managed database. Now env.py only
sets it when unset, and a new integration suite runs `alembic upgrade
head` against a real Postgres container per ADR-0016 (no create_all()).

tests/integration/postgres/conftest.py

@@ -0,0 +1,72 @@
+from collections.abc import AsyncIterator, Iterator
+
+import pytest
+import pytest_asyncio
+from alembic.command import upgrade
+from alembic.config import Config
+from sqlalchemy.ext.asyncio import AsyncEngine, AsyncSession, async_sessionmaker
+from testcontainers.community.postgres import PostgresContainer
+
+from src.config import PostgresSettings
+from src.infrastructure.postgres.database import create_engine
+
+
+def _alembic_config(database_url: str) -> Config:
+    config = Config("alembic.ini")
+    config.set_main_option("sqlalchemy.url", database_url)
+    return config
+
+
+def _settings_from_url(url: str) -> PostgresSettings:
+    # testcontainers returns postgresql+asyncpg://user:pass@host:port/db ;
+    # PostgresSettings builds its own dsn from parts, so parse the parts back out.
+    without_scheme = url.split("://", 1)[1]
+    creds, hostpart = without_scheme.split("@", 1)
+    user, password = creds.split(":", 1)
+    hostport, db = hostpart.split("/", 1)
+    host, port = hostport.split(":", 1)
+    return PostgresSettings(host=host, port=int(port), user=user, password=password, db=db)
+
+
+@pytest.fixture(scope="session")
+def postgres_container() -> Iterator[PostgresContainer]:
+    with PostgresContainer("postgres:17", driver="asyncpg") as container:
+        yield container
+
+
+@pytest.fixture(scope="session")
+def postgres_url(postgres_container: PostgresContainer) -> str:
+    return postgres_container.get_connection_url()
+
+
+@pytest.fixture(scope="session")
+def migrated_postgres_url(postgres_url: str) -> str:
+    """The container's URL, after Alembic has created the schema on it once."""
+    upgrade(_alembic_config(postgres_url), "head")
+    return postgres_url
+
+
+@pytest_asyncio.fixture(scope="session")
+async def postgres_engine(migrated_postgres_url: str) -> AsyncIterator[AsyncEngine]:
+    engine = create_engine(_settings_from_url(migrated_postgres_url))
+    try:
+        yield engine
+    finally:
+        await engine.dispose()
+
+
+@pytest_asyncio.fixture
+async def db_session(postgres_engine: AsyncEngine) -> AsyncIterator[AsyncSession]:
+    """One session per test, bound to a rolled-back outer transaction.
+
+    Isolates each test's writes (ADR-0016: isolate data per test) without
+    needing a fresh container or unique keys per test.
+    """
+    async with postgres_engine.connect() as connection:
+        outer_transaction = await connection.begin()
+        sessionmaker = async_sessionmaker(
+            bind=connection, expire_on_commit=False, join_transaction_mode="create_savepoint"
+        )
+        async with sessionmaker() as session:
+            yield session
+        await outer_transaction.rollback()