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/services/history_service.py

@@ -15,7 +15,7 @@ from typing import TYPE_CHECKING
 
 import structlog
 
-from app.models.ban import TIME_RANGE_SECONDS, BanOrigin, TimeRange
+from app.models.ban import BanOrigin, TimeRange
 from app.services import geo_service
 
 if TYPE_CHECKING:
@@ -35,6 +35,7 @@ from app.repositories import history_archive_repo as default_history_archive_rep
 from app.services.fail2ban_metadata_service import default_fail2ban_metadata_service
 from app.utils.constants import DEFAULT_PAGE_SIZE, MAX_PAGE_SIZE
 from app.utils.fail2ban_db_utils import parse_data_json, ts_to_iso
+from app.utils.time_utils import since_unix
 
 log: structlog.stdlib.BoundLogger = structlog.get_logger()
 
@@ -43,25 +44,11 @@ async def get_fail2ban_db_path(socket_path: str) -> str:
     """Return the fail2ban database path using the shared metadata cache."""
     return await default_fail2ban_metadata_service.get_db_path(socket_path)
 
-
 # ---------------------------------------------------------------------------
-# Constants
+# Internal Helpers
 # ---------------------------------------------------------------------------
 
 
-def _since_unix(range_: TimeRange) -> int:
-    """Return the Unix timestamp for the start of the given time window.
-
-    Args:
-        range_: One of the supported time-range presets.
-
-    Returns:
-        Unix timestamp (seconds since epoch) equal to *now − range_*.
-    """
-    seconds: int = TIME_RANGE_SECONDS[range_]
-    return int(datetime.now(tz=UTC).timestamp()) - seconds
-
-
 async def _resolve_geo_info(
     ip: str,
     *,
@@ -199,7 +186,7 @@ async def list_history(
     # Build WHERE clauses dynamically.
     since: int | None = None
     if range_ is not None:
-        since = _since_unix(range_)
+        since = since_unix(range_)
 
     db_path: str = await get_fail2ban_db_path(socket_path)
     log.info(