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/app/settings.py

@@ -1,11 +1,27 @@
-from pydantic_settings import BaseSettings
+from pathlib import Path
+
+from pydantic_settings import BaseSettings, SettingsConfigDict
+
+# Repo-root .env (shared with the compose stack). Real environment variables
+# always take precedence over .env values.
+REPO_ROOT = Path(__file__).resolve().parent.parent.parent
 
 
 class Settings(BaseSettings):
+    model_config = SettingsConfigDict(env_file=REPO_ROOT / ".env", extra="ignore")
+
     version: str = "0.1.0"
     debug: bool = False
 
-    model_config = {"env_prefix": "KIRCHE_"}
+    database_url: str = "postgresql+psycopg://kirche:kirche@localhost:5432/kirche_einfach"
+    test_database_url: str = (
+        "postgresql+psycopg://kirche:kirche@localhost:5432/kirche_einfach_test"
+    )
+
+    langflow_url: str = "http://localhost:7860"
+    langflow_api_key: str = ""
+    lfx_url: str = "http://localhost:6004"
+    llm_api_key: str = ""
 
 
 settings = Settings()