Fix: Consolidate divergent _since_unix implementations (T-09)

Consolidate the two divergent implementations of _since_unix from ban_service.py
and history_service.py into a single shared utility function in time_utils.py.

Changes:
- Move _since_unix to app/utils/time_utils.py with consistent time.time() approach
- Move TIME_RANGE_SLACK_SECONDS constant to app/utils/constants.py
- Update ban_service.py to import since_unix from time_utils
- Update history_service.py to import since_unix from time_utils
- Both services now use the same window boundary calculation with 60-second slack
- Add comprehensive tests for the shared since_unix function
- Document timestamp handling rationale in Backend-Development.md

This ensures dashboard and history queries return consistent row counts for the
same time range by using the same timestamp calculation and slack window across
all services.

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

backend/tests/test_services/test_time_utils.py

@@ -1,8 +1,17 @@
 """Tests for app.utils.time_utils."""
 
 import datetime
+import time
 
-from app.utils.time_utils import add_minutes, hours_ago, is_expired, utc_from_timestamp, utc_now
+from app.utils.time_utils import (
+    add_minutes,
+    hours_ago,
+    is_expired,
+    since_unix,
+    utc_from_timestamp,
+    utc_now,
+)
+from app.utils.constants import TIME_RANGE_SLACK_SECONDS
 
 
 class TestUtcNow:
@@ -77,3 +86,51 @@ class TestHoursAgo:
         expected_min = before - datetime.timedelta(hours=1, seconds=1)
         expected_max = after - datetime.timedelta(hours=1) + datetime.timedelta(seconds=1)
         assert expected_min <= result <= expected_max
+
+
+class TestSinceUnix:
+    """Tests for :func:`since_unix`."""
+
+    def test_since_unix_24h_returns_unix_timestamp(self) -> None:
+        """Verify since_unix returns an integer timestamp."""
+        result = since_unix("24h")
+        assert isinstance(result, int)
+
+    def test_since_unix_24h_is_roughly_24_hours_ago(self) -> None:
+        """Verify 24h preset returns a timestamp ~24 hours in the past."""
+        before = int(time.time())
+        result = since_unix("24h")
+        after = int(time.time())
+
+        # Allow 1 second tolerance for execution time
+        expected_min = before - (24 * 3600) - TIME_RANGE_SLACK_SECONDS - 1
+        expected_max = after - (24 * 3600) - TIME_RANGE_SLACK_SECONDS + 1
+
+        assert expected_min <= result <= expected_max
+
+    def test_since_unix_7d_is_roughly_7_days_ago(self) -> None:
+        """Verify 7d preset returns a timestamp ~7 days in the past."""
+        before = int(time.time())
+        result = since_unix("7d")
+        after = int(time.time())
+
+        # Allow 1 second tolerance for execution time
+        expected_min = before - (7 * 24 * 3600) - TIME_RANGE_SLACK_SECONDS - 1
+        expected_max = after - (7 * 24 * 3600) - TIME_RANGE_SLACK_SECONDS + 1
+
+        assert expected_min <= result <= expected_max
+
+    def test_since_unix_includes_slack_window(self) -> None:
+        """Verify 60-second slack is included in all presets."""
+        now = int(time.time())
+        result = since_unix("24h")
+
+        # Verify slack is included: result should be (now - 24h - 60s)
+        # within tolerance
+        diff_without_slack = now - result
+        expected_without_slack = 24 * 3600
+        actual_slack = diff_without_slack - expected_without_slack
+
+        # The slack should be ~60 seconds
+        assert actual_slack >= TIME_RANGE_SLACK_SECONDS - 1
+        assert actual_slack <= TIME_RANGE_SLACK_SECONDS + 1