Dashboard coverage: count 5-minute slots with data, not readings vs the recent interval

Coverage divided each day's reading count by 1440/median-recent-interval, so days logged at a
slower pace (100 s, 120 s) showed as 50-60% complete once uploads sped up to 60 s. Each day now
records which of its 288 five-minute slots have a reading (backfilled from raw files on startup),
so coverage is independent of upload interval. Older days without slot data fall back to the old maths.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

app/public/dashboard-ui.js

@@ -301,7 +301,13 @@ function medianIntervalMin() {
   d.sort((a, b) => a - b);
   return d[d.length >> 1] / 60000;
 }
-function coverageFor(row, perDay) { return row && perDay ? Math.min(100, (row.n / perDay) * 100) : null; }
+// Share of the day's 288 five-minute slots that have at least one reading, so it doesn't matter whether the
+// station reports every 60, 100 or 120 s. Days saved before slots were tracked fall back to reading count.
+function coverageFor(row, perDay) {
+  if (!row) return null;
+  if (row.slots != null) return Math.min(100, (row.slots / 288) * 100);
+  return perDay ? Math.min(100, (row.n / perDay) * 100) : null;
+}
 
 function renderHealthCard() {
   const host = $('healthCard');
@@ -311,7 +317,8 @@ function renderHealthCard() {
   const interval = medianIntervalMin();
   const perDay = interval ? 1440 / interval : null;
   const past = DAILY.filter((d) => d.d !== todayStr()).slice(-30);
-  const cov = perDay && past.length ? mean(past.map((d) => coverageFor(d, perDay))) : null;
+  const covVals = past.map((d) => coverageFor(d, perDay)).filter((v) => v != null);
+  const cov = covVals.length ? mean(covVals) : null;
   const t = todayRow();
   host.innerHTML = `<div class="ui-card"><h3>Station Health</h3><div class="kv">
     <div><b><span class="pill ${ok ? 'ok' : 'warn'}">${ok ? 'Online' : 'Delayed'}</span></b><span>Last report</span><small>${ageMin <= 0 ? 'just now' : ageMin + ' min ago'}</small></div>
@@ -530,8 +537,8 @@ function renderInsights() {
   const interval = medianIntervalMin();
   const perDay = interval ? 1440 / interval : null;
   const cov = past.slice(-45);
-  if (perDay) {
-    const vals = cov.map((d) => +coverageFor(d, perDay).toFixed(0));
+  if (cov.length) {
+    const vals = cov.map((d) => +(coverageFor(d, perDay) ?? 0).toFixed(0));
     const ds = barDataset('Coverage %', vals, c.cool);
     ds.backgroundColor = vals.map((v) => (v < 90 ? c.accent + 'bb' : c.cool + 'bb'));
     makeChart('inCoverage', 'bar', [ds], cov.map((d) => prettyDay(d.d)), '%', { scales: { y: { min: 0, max: 100 } } });