Add alert history dashboard endpoint and UI panel

GET /api/dashboard/alert-history?days=N reads daily .jsonl files from
data/alerts/ and returns entries newest-first. Dashboard gets a lazy-loaded
collapsible panel with a table showing time, type (NWS/threshold badge),
event name, and details, with 7d/30d/90d range selector.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/server.js

@@ -1404,6 +1404,26 @@ app.get('/api/station/trend', requireDashboardAuth, (_req, res) => {
   });
 });
 
+// Alert history log
+app.get('/api/dashboard/alert-history', requireDashboardAuth, (req, res) => {
+  const days = Math.min(90, Math.max(1, parseInt(req.query.days) || 30));
+  const results = [];
+  const now = Date.now();
+  for (let i = 0; i < days; i++) {
+    const day = new Date(now - i * 86400000).toISOString().slice(0, 10);
+    const file = path.join(ALERTS_DIR, `${day}.jsonl`);
+    if (!fs.existsSync(file)) continue;
+    try {
+      const lines = fs.readFileSync(file, 'utf8').split('\n').filter(Boolean);
+      for (const line of lines) {
+        try { results.push(JSON.parse(line)); } catch {}
+      }
+    } catch {}
+  }
+  results.sort((a, b) => b.ts - a.ts);
+  res.json(results);
+});
+
 app.listen(PORT, () => {
   console.log(`Weather app running on port ${PORT}`);
   if (dashAuth) {