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>

db.py

@@ -269,6 +269,37 @@ def deactivate_suppression(sup_id: int) -> None:
             )
 
 
+def cleanup_expired_suppressions() -> int:
+    """Mark expired time-limited suppressions as inactive. Returns count deactivated."""
+    with get_conn() as conn:
+        with conn.cursor() as cur:
+            cur.execute(
+                """UPDATE suppression_rules
+                   SET active=FALSE
+                   WHERE active=TRUE AND expires_at IS NOT NULL AND expires_at <= NOW()"""
+            )
+            n = cur.rowcount
+    if n:
+        logger.info(f'Deactivated {n} expired suppression(s)')
+    return n
+
+
+def purge_old_resolved_events(days: int = 90) -> int:
+    """Delete resolved events older than `days` days. Returns count deleted."""
+    with get_conn() as conn:
+        with conn.cursor() as cur:
+            cur.execute(
+                """DELETE FROM network_events
+                   WHERE resolved_at IS NOT NULL
+                   AND resolved_at < DATE_SUB(NOW(), INTERVAL %s DAY)""",
+                (days,),
+            )
+            n = cur.rowcount
+    if n:
+        logger.info(f'Purged {n} old resolved event(s) (>{days}d)')
+    return n
+
+
 def check_suppressed(suppressions: list, target_type: str, target_name: str, target_detail: str = '') -> bool:
     """Check suppression against a pre-loaded list (avoids per-call DB queries)."""
     for s in suppressions: