Harden template for production: add comments, fix bugs, add lock file

- Add inline comments to docker-compose.yml explaining the 5 key rules
  (expose vs ports, DB host, DB name persistence, no Traefik labels)
- Add comments to Dockerfile explaining multi-stage build, layer caching,
  and why .dockerignore excludes client/dist
- Add comments to .dockerignore explaining each exclusion
- Fix dev script: use nodemon (auto-restart) instead of node for server.js
- Add postinstall script to auto-install client deps (cd client && npm install)
- Fix SPA fallback: bare return → next() to prevent hanging requests
- Add root package-lock.json for deterministic server dependency installs
- Remove committed tsconfig.tsbuildinfo build artifact, add *.tsbuildinfo to .gitignore
- Update README: simpler install (npm install handles everything), reference SAAC_DEPLOYMENT.md,
  use npx instead of pnpm dlx for shadcn components

.dockerignore

@@ -1,6 +1,13 @@
+# Dependencies — installed inside the container via npm install
 node_modules
 client/node_modules
+
+# Built React output — the Dockerfile builds this fresh inside the container
+# using a multi-stage build (COPY --from=builder). Excluding it here prevents
+# stale local builds from leaking into the Docker build context.
 client/dist
+
+# Files not needed in the Docker image
 npm-debug.log
 .git
 .gitignore