fix: Update systemd services to use skyview-adsb user consistently

- Change User and Group from 'skyview' to 'skyview-adsb' in service files
- Update ReadWritePaths in database update service accordingly
- Add external data source configuration to example config

🤖 Generated with [Claude Code](https://claude.ai/code)

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

debian/lib/systemd/system/skyview-database-update.service

@@ -6,8 +6,8 @@ Wants=network-online.target
 
 [Service]
 Type=oneshot
-User=skyview
-Group=skyview
+User=skyview-adsb
+Group=skyview-adsb
 ExecStart=/usr/bin/skyview-data update
 StandardOutput=journal
 StandardError=journal
@@ -17,7 +17,7 @@ NoNewPrivileges=true
 PrivateTmp=true
 ProtectSystem=strict
 ProtectHome=true
-ReadWritePaths=/var/lib/skyview /tmp
+ReadWritePaths=/var/lib/skyview-adsb /tmp
 ProtectKernelTunables=true
 ProtectKernelModules=true
 ProtectControlGroups=true