Unify three coexisting time-display styles (raw datetime repr, bespoke
inline math, route-side humanize_delta) behind a single timeago Jinja
filter returning a <time> element with relative label and UTC tooltip.
Symmetric past/future ladder with second precision and day-month-year
fallback >7d. Naive-datetime DB-column cleanup tracked as a follow-up.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
