Add tests for since timestamp accuracy in ban_service

- test_since_unix_returns_utc_epoch: validates since_unix('24h') returns UTC epoch
- test_ban_trend_since_is_within_expected_range: validates 23h-ago ban falls in 24h+slack window

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/tests/test_services/test_time_utils.py

@@ -134,3 +134,15 @@ class TestSinceUnix:
         # The slack should be ~60 seconds
         assert actual_slack >= TIME_RANGE_SLACK_SECONDS - 1
         assert actual_slack <= TIME_RANGE_SLACK_SECONDS + 1
+
+    def test_since_unix_returns_utc_epoch(self) -> None:
+        """``since_unix('24h')`` returns a value within 24h + 60s of ``time.time()``."""
+        before = int(time.time())
+        result = since_unix("24h")
+        after = int(time.time())
+
+        # Allow 2 second tolerance for execution time
+        expected_min = before - (24 * 3600) - TIME_RANGE_SLACK_SECONDS - 2
+        expected_max = after - (24 * 3600) - TIME_RANGE_SLACK_SECONDS + 2
+
+        assert expected_min <= result <= expected_max