New features: stale banner, tab title alerts, health checks, DB housekeeping

static/app.js:
- Browser tab title updates to show alert count: '(3 CRIT) GANDALF' or '(2 WARN) GANDALF'
- Stale monitoring banner: injected into .main if last_check > 15 min old,
  warns operator that the monitor daemon may be down

static/style.css:
- .stale-banner: amber top-border warning strip

app.py:
- /health now checks DB connectivity and monitor freshness (last_check age)
  Returns 503 + degraded status if DB unreachable or monitor stale >20min

db.py:
- cleanup_expired_suppressions(): marks time-limited suppressions inactive when
  expires_at <= NOW() (was only filtered in SELECTs, never marked inactive)
- purge_old_resolved_events(days=90): deletes old resolved events to prevent
  unbounded table growth

monitor.py:
- Calls cleanup_expired_suppressions() and purge_old_resolved_events() each cycle

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

static/app.js

@@ -49,6 +49,37 @@ function updateStatusBar(summary, lastCheck) {
 
   const lc = document.getElementById('last-check');
   if (lc && lastCheck) lc.textContent = lastCheck;
+
+  // Update browser tab title with alert count
+  const critCount = summary.critical || 0;
+  const warnCount = summary.warning || 0;
+  if (critCount) {
+    document.title = `(${critCount} CRIT) GANDALF`;
+  } else if (warnCount) {
+    document.title = `(${warnCount} WARN) GANDALF`;
+  } else {
+    document.title = 'GANDALF';
+  }
+
+  // Stale data banner: warn if last_check is older than 15 minutes
+  let staleBanner = document.getElementById('stale-banner');
+  if (lastCheck) {
+    // last_check format: "2026-03-14 14:14:21 UTC"
+    const checkAge = (Date.now() - new Date(lastCheck.replace(' UTC', 'Z').replace(' ', 'T'))) / 1000;
+    if (checkAge > 900) { // 15 minutes
+      if (!staleBanner) {
+        staleBanner = document.createElement('div');
+        staleBanner.id = 'stale-banner';
+        staleBanner.className = 'stale-banner';
+        document.querySelector('.main').prepend(staleBanner);
+      }
+      const mins = Math.floor(checkAge / 60);
+      staleBanner.textContent = `⚠ Monitoring data is stale — last check was ${mins} minute${mins !== 1 ? 's' : ''} ago. The monitor daemon may be down.`;
+      staleBanner.style.display = '';
+    } else if (staleBanner) {
+      staleBanner.style.display = 'none';
+    }
+  }
 }
 
 function updateHostGrid(hosts) {