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/conftest.py

@@ -0,0 +1,69 @@
+"""Shared pytest fixtures.
+
+Requires TEST_DATABASE_URL (a dedicated Postgres database, e.g. the
+`kirche_einfach_test` database from the compose stack). The URL is mapped
+onto DATABASE_URL *before* any app module is imported, so settings and the
+alembic env both target the test database. Never point this at production.
+"""
+
+import os
+from pathlib import Path
+
+import pytest
+
+TEST_DB_URL = os.environ.get("TEST_DATABASE_URL")
+if not TEST_DB_URL:
+    raise RuntimeError(
+        "TEST_DATABASE_URL is not set. Refusing to run tests without "
+        "an explicit test database (see .env.example)."
+    )
+
+# Must precede all `app.*` imports: Settings is instantiated at import time.
+os.environ["DATABASE_URL"] = TEST_DB_URL
+
+BACKEND_DIR = Path(__file__).resolve().parent.parent
+
+
+@pytest.fixture(scope="session")
+def db_engine():
+    """Engine on the test DB, migrated to head once per session."""
+    from alembic import command as alembic_command
+    from alembic.config import Config
+
+    from app import db as app_db
+
+    app_db.reset_engine()
+    engine = app_db.get_engine()
+
+    cfg = Config()
+    cfg.set_main_option("script_location", str(BACKEND_DIR / "migrations"))
+    alembic_command.upgrade(cfg, "head")
+
+    yield engine
+
+    engine.dispose()
+    app_db.reset_engine()
+
+
+@pytest.fixture()
+def session(db_engine):
+    """Function-scoped session; tables are truncated after each test."""
+    from sqlmodel import Session, SQLModel
+
+    with Session(db_engine) as s:
+        yield s
+    with db_engine.begin() as conn:
+        for table in reversed(SQLModel.metadata.sorted_tables):
+            conn.execute(table.delete())
+
+
+@pytest.fixture()
+async def client():
+    """ASGI test client for the FastAPI app."""
+    from httpx import ASGITransport, AsyncClient
+
+    from app.main import app
+
+    transport = ASGITransport(app=app)
+    async with AsyncClient(transport=transport, base_url="http://test") as c:
+        yield c