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/database_migrations/03-add_device_events/up.sql

@@ -0,0 +1,8 @@
+CREATE TABLE device_events(
+    id INTEGER PRIMARY KEY,
+    mac_address TEXT NOT NULL,
+    created_on TEXT NOT NULL,
+    event_type TEXT NOT NULL,
+    ipv4_address TEXT NOT NULL,
+    vendor TEXT NOT NULL
+);