feat: compose stack in devcontainer (db, langflow, lfx)

- docker-compose.yml with app/db/langflow/lfx services and healthchecks
- Dockerfile.lfx (stateless flow runtime on :6004)
- init-db.sql (langflow + test databases)
- devcontainer.json switched to compose mode with runServices
- setup.sh waits for compose services and requires .env

.env.example

@@ -15,6 +15,7 @@ LANGFLOW_SUPERUSER_PASSWORD=
 LANGFLOW_SECRET_KEY=
 # Single static key; validated against env (LANGFLOW_API_KEY_SOURCE=env), superuser scope.
 LANGFLOW_API_KEY=sk-
+LANGFLOW_DATABASE_URL=postgresql://kirche:<password>@db:5432/langflow
 
 # --- lfx (flow runtime) ---
 LFX_URL=http://lfx:6004