refactor: split json_helper.py into lib/ modules
- core/lib/util.py: shared utilities, ip_to_name, reverse_lookup, parse_since
- core/lib/events.py: fw_events, wg_events, follow_logs, event parsers
- core/lib/peers.py: peer_data, peer_transfer, peer_transfer_delta
- core/lib/activity.py: activity_aggregate
- json_helper.py: thin dispatcher importing from lib/
- events.py: --since, --filter-event, --filter-dest-ip/port query flags
- util.py: parse_since supporting relative (2h/7d) and EU/ISO date formats
1308f9e07a