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>