Without this, 'mix release' produced a tarball that had app.css/app.js
(so LiveView worked) but was missing cache_manifest.json and the digested
asset paths. Phoenix served the bare files OK, but the client-side
LiveView bootstrap timing was fragile: if a form was submitted before
the LiveSocket attached, the browser fell back to a native HTML GET,
producing bug-report URLs like /admin/hosts?host%5Bname%5D=repl.
Define a project releases/0 with a pre-assemble step that runs
assets.deploy, so minified + digested assets are baked into every
release tarball.
Also gitignore digested priv/static artifacts so dev-time byproducts
don't pollute commits.
