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-adsb.service

@@ -8,6 +8,9 @@ Wants=network.target
 Type=simple
 User=skyview-adsb
 Group=skyview-adsb
+# Update database before starting main service  
+ExecStartPre=/usr/bin/skyview-data -config /etc/skyview-adsb/config.json update
+TimeoutStartSec=300
 ExecStart=/usr/bin/skyview -config /etc/skyview-adsb/config.json
 WorkingDirectory=/var/lib/skyview-adsb
 StandardOutput=journal

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