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/app/utils/constants.py

@@ -65,6 +65,9 @@ TIME_RANGE_HOURS: Final[dict[str, int]] = {
     TIME_RANGE_365D: 365 * 24,
 }
 
+TIME_RANGE_SLACK_SECONDS: Final[int] = 60
+"""Clock drift and test seeding tolerance for timestamp comparisons."""
+
 # ---------------------------------------------------------------------------
 # Pagination
 # ---------------------------------------------------------------------------

backend/app/utils/time_utils.py

@@ -7,6 +7,7 @@ for working with time throughout the backend.
 """
 
 import datetime
+import time
 
 
 def utc_now() -> datetime.datetime:
@@ -65,3 +66,31 @@ def hours_ago(hours: int) -> datetime.datetime:
         Timezone-aware UTC :class:`datetime.datetime`.
     """
     return utc_now() - datetime.timedelta(hours=hours)
+
+
+def since_unix(range_: str) -> int:
+    """Return the Unix timestamp for the start of a time-range window.
+
+    Uses :func:`time.time` (always UTC epoch seconds on all platforms) to be
+    consistent with how fail2ban stores ``timeofban`` values in its SQLite
+    database. fail2ban records :func:`time.time()` values directly, so using
+    a timezone-aware :func:`datetime.datetime.now`\\ ``(UTC).timestamp()``
+    would theoretically produce the same result but using :func:`time.time`
+    avoids any timezone-aware datetime pitfalls on misconfigured systems.
+
+    A 60-second slack window is applied to accommodate clock drift and
+    test seeding delays. This ensures consistent query windows across services
+    (e.g., dashboard vs. history).
+
+    Args:
+        range_: One of the supported time-range presets (e.g., ``"24h"``).
+
+    Returns:
+        Unix timestamp (seconds since epoch) representing the start of the
+        time window: *now − range_ − slack*.
+    """
+    from app.models.ban import TIME_RANGE_SECONDS  # noqa: F401
+    from app.utils.constants import TIME_RANGE_SLACK_SECONDS
+
+    seconds: int = TIME_RANGE_SECONDS[range_]
+    return int(time.time()) - seconds - TIME_RANGE_SLACK_SECONDS