test(monitors): take "12 minutes ago" from the render's own clock (#5121)

NOW is fixed when the module is imported; by the time the detail page
rendered in CI seven seconds had passed, and |ago rightly said "12m 7s
ago". The page was right; the test's clock was stale.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/core/test_monitors_page.py

@@ -93,7 +93,7 @@ async def test_the_monitor_page_says_since_when_and_why(app):
         session["user_role"] = "admin"
         page = await render_template(
             "monitors/detail.html", monitor=m, host=None, latest=down, failures=[down], state="down",
-            since=NOW - timedelta(minutes=12), since_at_least=False, tls_days=None,
+            since=datetime.now(timezone.utc) - timedelta(minutes=12), since_at_least=False, tls_days=None,
             uptime={"24h": 93.0, "7d": None, "30d": 99.5},
             heartbeat=[{"state": "up", "title": "a"}, {"state": "down", "title": "b"}])
     assert '<a href="/monitors/">Monitors</a>' in page and "shop &lt;api&gt;" in page