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

.devcontainer/init-db.sql

@@ -0,0 +1,7 @@
+-- Runs once on first init of the postgres-data volume.
+-- The main app database ($POSTGRES_DB) is created by the image itself.
+-- Langflow (authoring UI) gets its own database on the same instance.
+CREATE DATABASE langflow;
+
+-- Isolated database for the pytest suite (migrated via Alembic in conftest).
+CREATE DATABASE kirche_einfach_test;