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/Dockerfile.lfx

@@ -0,0 +1,19 @@
+# Stateless Langflow flow runtime (LFX). Serves versioned flow JSON files
+# deployed by our backend via POST /flows/upload/ (persisted in /flows).
+# LFX stays on the internal compose network; our FastAPI is the only
+# public surface and proxies run/stream calls.
+FROM python:3.12-slim
+
+ENV PYTHONUNBUFFERED=1 \
+    PIP_NO_CACHE_DIR=1
+
+# lfx = flow runtime; requests = convenience for custom-component code.
+RUN pip install "lfx==1.12.0" requests
+
+RUN mkdir -p /flows
+VOLUME /flows
+EXPOSE 6004
+
+# Serve the /flows dir (uploaded flows persist there via --flow-dir).
+# Flags verified against `lfx serve --help` (lfx 1.12.0).
+CMD ["lfx", "serve", "--host", "0.0.0.0", "--port", "6004", "--flow-dir", "/flows", "/flows"]