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/devcontainer.json

@@ -1,15 +1,10 @@
 {
   "name": "kirche-einfach",
-  "build": {
-    "dockerfile": "Dockerfile",
-    "args": {
-      "FORGEJO_USERNAME": "jonas"
-    }
-  },
+  "dockerComposeFile": "docker-compose.yml",
+  "service": "app",
+  "runServices": ["db", "langflow", "lfx"],
   "mounts": [
-    "source=${localEnv:USERPROFILE}/.ssh,target=/mnt/host-ssh,type=bind,readonly",
-    "source=kirche-einfach-venv,target=/workspaces/kirche-einfach/backend/.venv,type=volume",
-    "source=kirche-einfach-node-modules,target=/workspaces/kirche-einfach/frontend/node_modules,type=volume"
+    "source=${localEnv:USERPROFILE}/.ssh,target=/mnt/host-ssh,type=bind,readonly"
   ],
   "remoteUser": "vscode",
   "containerEnv": {