Version CSS and JS URLs in production builds

[karim-semmoud]

Summary

After a deploy, the live site kept serving the previous site.css until a hard refresh. GitHub Pages caches assets, and the CSS and JS URLs never changed between builds.

• Add Soonex.Public.asset/1, which appends ?v=<version> when :asset_version is configured.
• config/prod.exs sets the version to the first 12 characters of GITHUB_SHA (provided by GitHub Actions), or the build time in seconds outside Actions.
• The root layout loads site.css and site.js through asset/1. Dev and test stay unversioned.
• esbuild's split chunks are already content-hashed, so only the entry files need the query string.
• Bump to 0.3.1 with a CHANGELOG entry.

Test plan

• Under prod config, asset("/css/site.css") returns /soonex/css/site.css?v=<timestamp>, and with GITHUB_SHA set it returns ?v=<sha>.
• In dev, asset("/css/site.css") returns /css/site.css.
• mix compile --warnings-as-errors and mix credo --strict are clean.