feat(admin): profile freshness panel in data-quality (#81 phase B.4)

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>

services/api/src/profile/builder.ts

@@ -120,6 +120,58 @@ export interface ProfileFeatureView {
   description: string;
 }
 
+export interface FeatureFreshnessSummary {
+  feature: string;
+  ttlSec: number;
+  /** Distinct users with tip activity in the last 30 days. */
+  totalEligible: number;
+  /** Eligible users with no row stored at all for this feature. */
+  missing: number;
+  /** Eligible users whose stored row is past its TTL. */
+  stale: number;
+}
+
+/**
+ * Per-feature staleness summary across all eligible users (anyone with a tip
+ * served in the last 30 days). Used by `/admin/data-quality` so the admin can
+ * spot features that are systematically stale or never computed.
+ *
+ * Hand-written SQL because this joins (eligible_users LEFT JOIN profile_rows)
+ * with conditional aggregations — drizzle's query builder is more pain than
+ * value here, and the column allowlist is the registry.
+ */
+export function summarizeProfileFreshness(): FeatureFreshnessSummary[] {
+  return FEATURES.map((f) => {
+    const row = rawSqlite
+      .prepare(`
+        WITH eligible AS (
+          SELECT DISTINCT user_id
+          FROM tip_views
+          WHERE served_at >= datetime('now', '-30 days')
+        )
+        SELECT
+          COUNT(*) AS total_eligible,
+          SUM(CASE WHEN upf.user_id IS NULL THEN 1 ELSE 0 END) AS missing,
+          SUM(CASE WHEN upf.user_id IS NOT NULL
+                    AND upf.ttl_sec > 0
+                    AND (julianday('now') - julianday(upf.updated_at)) * 86400.0 > upf.ttl_sec
+                   THEN 1 ELSE 0 END) AS stale
+        FROM eligible e
+        LEFT JOIN user_profile_features upf
+          ON upf.user_id = e.user_id AND upf.name = ?
+      `)
+      .get(f.name) as { total_eligible: number; missing: number; stale: number } | undefined;
+
+    return {
+      feature: f.name,
+      ttlSec: f.ttlSec,
+      totalEligible: Number(row?.total_eligible ?? 0),
+      missing: Number(row?.missing ?? 0),
+      stale: Number(row?.stale ?? 0),
+    };
+  });
+}
+
 /**
  * Inspection helper for the admin UI: returns one row per registered feature,
  * joining stored value + metadata. No compute — surface staleness; rebuild is