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/test_migrations.py

@@ -0,0 +1,23 @@
+import pytest
+from sqlalchemy import inspect
+from sqlalchemy.ext.asyncio import AsyncEngine
+
+pytestmark = [pytest.mark.integration, pytest.mark.postgres, pytest.mark.asyncio]
+
+EXPECTED_TABLES = {
+    "tenants",
+    "api_keys",
+    "source_files",
+    "ingestion_jobs",
+    "ingestion_job_events",
+    "alembic_version",
+}
+
+
+async def test_migrations_create_schema_from_empty_database(postgres_engine: AsyncEngine) -> None:
+    async with postgres_engine.connect() as connection:
+        table_names = await connection.run_sync(
+            lambda sync_conn: inspect(sync_conn).get_table_names()
+        )
+
+    assert EXPECTED_TABLES.issubset(set(table_names))