base.html stamps the same asset_version() value onto both app.css and
app.js's <link>/<script> tags, but the value was computed from app.css's
mtime alone. A JS-only deploy left it unchanged, so app.js's URL stayed
byte-identical to the previous deploy -- nothing forced a browser holding
that URL cached to refetch it.
Extracted into _static_asset_version(web_dir), keyed to the newer of the
two files' mtimes, and unit-tested directly (a JS-only mtime bump now
changes the value; both-missing degrades to "0").
Simone
3d0c32b488