fix: UTC timezone suffix missing from all isoformat() timestamp outputs

db.py returned all datetime columns (first_seen, last_seen, resolved_at,
created_at, expires_at) as bare ISO strings like "2026-03-14T14:14:21"
with no timezone marker. Per the ECMAScript spec, new Date() on a
datetime string without timezone treats it as LOCAL time, not UTC.
This made lt.time.ago() and stale-detection wrong for any user whose
browser is not in UTC — event ages and stale warnings would be off by
the client's UTC offset.

monitor.py had the same issue on the network_snapshot 'updated' field.

Fix: append 'Z' to all isoformat() calls (UTC datetimes confirmed by
MySQL server timezone and _now_utc() pattern used throughout codebase).

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

monitor.py

@@ -918,7 +918,7 @@ class NetworkMonitor:
         return {
             'hosts': hosts,
             'unifi': display_unifi,
-            'updated': datetime.utcnow().isoformat(),
+            'updated': datetime.utcnow().isoformat() + 'Z',
         }
 
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