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>

monitor.py

@@ -916,6 +916,10 @@ class NetworkMonitor:
 
                 self._process_ping_hosts(suppressions)
 
+                # Housekeeping: deactivate expired suppressions and purge old resolved events
+                db.cleanup_expired_suppressions()
+                db.purge_old_resolved_events(days=90)
+
                 logger.info('Network check cycle complete')
 
             except Exception as e: