Plug.Static with gzip: true serves .gz files when they exist
(created by phx.digest) and falls back to uncompressed otherwise.
The previous code_reloading? guard was needlessly conservative —
in dev without digested assets, there are no .gz files so nothing
changes. With the lighthouse task building prod assets, this gives
realistic transfer sizes (42KB JS vs 135KB).
Mobile lighthouse scores jump from 95-97 to 99-100.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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