Add finish photo URL to notification templates (Issue #126)

- Add {finish_photo_url} template variable for print_complete, print_failed,
  print_stopped events
- Photo capture now completes before notification is sent (ensures image exists)
- Add External URL setting in Settings → Network (auto-detects from browser)
- Full URL constructed using external_url setting for external services
- Fix Telegram Markdown parsing error when messages contain URLs
- Add backend schema for external_url setting
- Add unit test for finish_photo_url variable passing

frontend/src/api/client.ts

@@ -617,6 +617,8 @@ export interface AppSettings {
   mqtt_password: string;
   mqtt_topic_prefix: string;
   mqtt_use_tls: boolean;
+  // External URL for notifications
+  external_url: string;
   // Home Assistant integration
   ha_enabled: boolean;
   ha_url: string;