feat: flow engine data layer with tests

- SQLModel tables: flow_definitions/versions/runs, approval_tasks
- Alembic setup with autogenerate (sqlmodel types fixed via script.py.mako)
- Settings extended (shared root .env), db engine/session helpers
- pytest suite: conftest with isolated test DB, 5 model tests green

backend/tests/test_models.py

@@ -0,0 +1,82 @@
+"""Model-level tests: defaults, constraints, JSON roundtrip, linkage."""
+
+import pytest
+from sqlalchemy.exc import IntegrityError
+from sqlmodel import select
+
+from app.models import ApprovalTask, FlowDefinition, FlowRun, FlowVersion
+
+
+def test_flow_definition_defaults(session):
+    d = FlowDefinition(name="bauvorhaben")
+    session.add(d)
+    session.commit()
+    session.refresh(d)
+    assert d.id is not None
+    assert d.status == "active"
+    assert d.continuation_flow_id is None
+    assert d.created_at is not None
+    assert d.created_at.tzinfo is not None
+
+
+def test_flow_definition_name_unique(session):
+    session.add(FlowDefinition(name="doppelt"))
+    session.commit()
+    session.add(FlowDefinition(name="doppelt"))
+    with pytest.raises(IntegrityError):
+        session.commit()
+    session.rollback()
+
+
+def test_flow_version_unique_per_flow(session):
+    d = FlowDefinition(name="v-flow")
+    session.add(d)
+    session.commit()
+    session.add(FlowVersion(flow_id=d.id, version_no=1, author="a"))
+    session.commit()
+    session.add(FlowVersion(flow_id=d.id, version_no=1, author="b"))
+    with pytest.raises(IntegrityError):
+        session.commit()
+    session.rollback()
+    session.add(FlowVersion(flow_id=d.id, version_no=2, author="b"))
+    session.commit()
+
+
+def test_flow_version_json_roundtrip(session):
+    payload = {"data": {"nodes": [{"id": "n1"}], "edges": []}, "name": "x"}
+    d = FlowDefinition(name="json-flow")
+    session.add(d)
+    session.commit()
+    v = FlowVersion(flow_id=d.id, version_no=1, langflow_json=payload)
+    session.add(v)
+    session.commit()
+    session.refresh(v)
+    assert v.langflow_json == payload
+    assert v.deployed_at is None
+    assert v.lfx_flow_id is None
+
+
+def test_run_and_task_linkage_with_tenant_default(session):
+    d = FlowDefinition(name="linked")
+    session.add(d)
+    session.commit()
+    v = FlowVersion(flow_id=d.id, version_no=1)
+    session.add(v)
+    session.commit()
+    run = FlowRun(version_id=v.id, input={"a": 1})
+    session.add(run)
+    session.commit()
+    task = ApprovalTask(run_id=run.id, version_id=v.id, title="bitte pruefen")
+    session.add(task)
+    session.commit()
+
+    assert run.tenant_id == "default"
+    assert run.status == "running"
+    assert task.tenant_id == "default"
+    assert task.status == "open"
+
+    runs = session.exec(select(FlowRun).where(FlowRun.version_id == v.id)).all()
+    assert len(runs) == 1
+    tasks = session.exec(select(ApprovalTask).where(ApprovalTask.run_id == run.id)).all()
+    assert len(tasks) == 1
+    assert tasks[0].payload == {}