Apply event deduplication window to notifications and refine its key

Gate notification persistence and delivery on the device-event
deduplication window, not just the device_events table: record_event now
reports whether it recorded, and classify_new_device/classify_existing_device
suppress their changes when the sighting is a duplicate.

Treat an empty->value IP fill as not a change via a new ip_changed helper
(mirroring vendor_changed), so a device gaining its first address raises no
"changed" notification.

Key deduplication on (mac, scanner, event_type) instead of (mac, ip,
scanner), so repeated sightings of the same kind collapse regardless of the
reported address.

Also delete stale oott.db-wal/-shm in run_tests.sh to avoid init_db panics.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/run_tests.sh

@@ -1,3 +1,3 @@
 #!/bin/sh
-sudo rm -f oott.db
+sudo rm -f oott.db oott.db-wal oott.db-shm
 sudo CARGO_HOME=$HOME/.cargo cargo test -- --show-output --test-threads=1