Add device_events table and scan history API endpoint

Records a DeviceEvent row on every scan: NewDevice when a device is first
seen, DeviceSeen on subsequent scans. Exposes GET /api/devices/{mac}/events
with pagination, ordered by date descending.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/db.rs

@@ -1,3 +1,4 @@
+pub mod device_events;
 pub mod devices;
 pub mod error;
 pub mod notifications;