fix: Address CodeRabbit review feedback

- Node version 21 → 22 (matches frontend package.json engines)
- Add language specifier to README code block (MD040)
- Add error handling to cd command in poststart.sh (SC2164)
- Add PYTHONPATH to seed test data launch config
- Use docker compose pull instead of hardcoded image tags

.devcontainer/platform/devcontainer.json

@@ -15,7 +15,7 @@
     },
     "ghcr.io/devcontainers/features/github-cli:1": {},
     "ghcr.io/devcontainers/features/node:1": {
-      "version": "21",
+      "version": "22",
       "nodeGypDependencies": true
     },
     "ghcr.io/devcontainers/features/python:1": {