Slim the API image: node 22, non-root, no Chromium

With Chromium gone from the API container, the image can be modernised properly. It's currently node:18 (end of life since April 2025), runs as root, installs Chromium via apk, and needs tsconfig-paths at runtime because of how the build handles path aliases.

• Base image to node:22-slim, add a USER directive so the app doesn't run as root
• Remove the Chromium install and the PUPPETEER_* env vars
• Fix the build so path aliases are resolved at compile time (tsup, or tsc + tsc-alias) and the dist layout stops nesting dist/backend/src
• Pin the dev Dockerfiles to the same node version instead of node:latest