Adds a per-feature freshness summary to /admin/data-quality so the admin
can spot features that are systematically stale or never computed:
totalEligible — distinct users with tip_views in the last 30 days
missing — eligible users with no row stored for the feature
stale — eligible users whose stored row is past its TTL
Backend exposes summarizeProfileFreshness() in profile/builder.ts; one
query per feature joins eligible users LEFT JOIN profile rows.
Coverage = (eligible − missing − stale) / eligible, colored
green/yellow/red via the new PctGood helper (high-is-good, opposite of
the existing Pct used for missing-feature/stale-token rates).
Refs #81.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
alvis
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