fix: first server alert suppressed within 5min of boot; deflake mute-expiry test

CI (running in a fresh UTC VM) surfaced two latent issues the local
Europe/London + long-uptime environment was hiding:

1. UnraidSystemMonitor._rate_limited_alert used a sentinel of 0 for
   'never alerted'. time.monotonic() is seconds-since-boot, so on a
   freshly-booted host (monotonic < _ALERT_COOLDOWN=300) the first
   server alert (CPU temp / memory / CPU usage) was wrongly suppressed
   for the first ~5 minutes of uptime — exactly when a hot reboot would
   most need it. Default to -inf so the first alert always fires.
   Reproduced at 5s simulated uptime: 85C alert suppressed before fix.

2. test_same_day asserted 'tomorrow' not in a now+1h mute expiry, which
   is flaky in the last hour before midnight. Pin 'now' to midday.

Verified: full suite 1065 passed under TZ=UTC and TZ=Europe/London;
ruff + mypy clean.

Author: dervish666

src/unraid/monitors/system_monitor.py

@@ -141,7 +141,11 @@ async def check_once(self) -> dict[str, Any] | None:
     async def _rate_limited_alert(self, key: str, **kwargs: Any) -> None:
         """Send an alert only if cooldown has elapsed for this key."""
         now = time.monotonic()
-        last = self._last_alert_times.get(key, 0)
+        # Default to -inf (not 0) so the first alert always fires: on a
+        # freshly-booted host time.monotonic() can be < _ALERT_COOLDOWN, and a
+        # sentinel of 0 would wrongly suppress the very first alert within the
+        # first few minutes of uptime.
+        last = self._last_alert_times.get(key, float("-inf"))
         if now - last < _ALERT_COOLDOWN:
             logger.debug(f"Suppressing duplicate {key} alert (cooldown)")
             return

tests/test_formatting_utils.py

@@ -227,12 +227,17 @@ class TestFormatMuteExpiry:
     """Tests for format_mute_expiry."""
 
     def test_same_day(self):
+        from unittest.mock import patch
         from zoneinfo import ZoneInfo
 
         tz = ZoneInfo("Europe/London")
-        now = datetime.now(tz)
-        expiry = now + timedelta(hours=1)
-        result = format_mute_expiry(expiry)
+        # Pin "now" to midday so expiry (+1h) cannot roll into tomorrow.
+        # Using the real clock made this flaky in the last hour before midnight.
+        fixed_now = datetime(2026, 6, 2, 12, 0, tzinfo=tz)
+        with patch("src.utils.formatting.datetime") as mock_dt:
+            mock_dt.now.return_value = fixed_now
+            expiry = fixed_now + timedelta(hours=1)
+            result = format_mute_expiry(expiry)
         assert result.startswith("until ")
         assert "tomorrow" not in result