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_ban_service.py

@@ -934,6 +934,29 @@ class TestBanTrend:
             parsed = datetime.fromisoformat(bucket.timestamp)
             assert parsed.tzinfo is not None  # Must be timezone-aware (UTC)
 
+    async def test_ban_trend_since_is_within_expected_range(self, tmp_path: Path) -> None:
+        """``since`` value is within 24h + 60s slack of the current time."""
+        from app.utils.constants import TIME_RANGE_SLACK_SECONDS
+
+        now = int(time.time())
+        # Place a ban just inside the expected range: 23 hours ago.
+        # With 60s slack, since ≈ now - 24h - 60s, so 23h-ago ban should be included.
+        just_inside_range = now - (23 * 3600)
+        path = str(tmp_path / "test_since_range.sqlite3")
+        await _create_f2b_db(
+            path,
+            [{"jail": "sshd", "ip": "1.2.3.4", "timeofban": just_inside_range}],
+        )
+
+        with patch(
+            "app.services.ban_service.get_fail2ban_db_path",
+            new=AsyncMock(return_value=path),
+        ):
+            result = await ban_service.ban_trend("/fake/sock", "24h")
+
+        # Ban at 23h ago must appear (within 24h + 60s window).
+        assert sum(b.count for b in result.buckets) == 1
+
 
 # ---------------------------------------------------------------------------
 # bans_by_jail