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>

config.example.json

@@ -44,5 +44,31 @@
     "history_limit": 1000,
     "stale_timeout": 60,
     "update_rate": 1
+  },
+  "database": {
+    "path": "",
+    "max_history_days": 7,
+    "backup_on_upgrade": true,
+    "max_open_conns": 10,
+    "max_idle_conns": 5
+  },
+  "callsign": {
+    "enabled": true,
+    "cache_hours": 24,
+    "privacy_mode": false,
+    "sources": {
+      "openflights_airlines": {
+        "enabled": true,
+        "priority": 1
+      },
+      "openflights_airports": {
+        "enabled": true,
+        "priority": 2
+      },
+      "ourairports": {
+        "enabled": true,
+        "priority": 3
+      }
+    }
   }
 }