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>

app.py

@@ -391,8 +391,38 @@ def api_diagnose_poll(job_id: str):
 
 @app.route('/health')
 def health():
-    """Health check endpoint (no auth)."""
-    return jsonify({'status': 'ok', 'service': 'gandalf'})
+    """Health check endpoint (no auth). Checks DB and monitor freshness."""
+    checks = {}
+    overall = 'ok'
+
+    # DB connectivity
+    try:
+        db.get_state('last_check')
+        checks['db'] = 'ok'
+    except Exception as e:
+        checks['db'] = f'error: {e}'
+        overall = 'degraded'
+
+    # Monitor freshness: fail if last_check is older than 20 minutes
+    try:
+        last_check = db.get_state('last_check', '')
+        if last_check:
+            from datetime import datetime, timezone
+            ts = datetime.strptime(last_check, '%Y-%m-%d %H:%M:%S UTC').replace(tzinfo=timezone.utc)
+            age_s = (datetime.now(timezone.utc) - ts).total_seconds()
+            if age_s > 1200:
+                checks['monitor'] = f'stale ({int(age_s)}s since last check)'
+                overall = 'degraded'
+            else:
+                checks['monitor'] = f'ok ({int(age_s)}s ago)'
+        else:
+            checks['monitor'] = 'no data yet'
+    except Exception as e:
+        checks['monitor'] = f'error: {e}'
+        overall = 'degraded'
+
+    status_code = 200 if overall == 'ok' else 503
+    return jsonify({'status': overall, 'service': 'gandalf', 'checks': checks}), status_code
 
 
 if __name__ == '__main__':