Renames scanner.rs to arp_scanner.rs to future-proof for additional scanner
types. Renames the [timings] config section to [arp_scanner] and simplifies
field names (arp_sender_timeout -> sender_timeout, arp_scan_duration ->
scan_duration). Introduces arp_scanner_status module to track whether the
scan is actively running or sleeping, and exposes this via a new
GET /api/arp_scanner/status endpoint returning is_running,
running_for_seconds, and next_run_in_seconds.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Purges records older than a configurable window (default 365d) once per day.
Also adds a composite index on device_events(mac_address, created_on DESC) for
efficient scan history queries at scale, and fixes a non-deterministic pagination
order by adding id as a tiebreaker to ORDER BY.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
