Add sync engine, web UI, Docker setup, and tests

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -0,0 +1,20 @@
+services:
+  outline-sync-ui:
+    build: .
+    container_name: outline-sync-ui
+    restart: unless-stopped
+    networks:
+      - domnet
+    ports:
+      - "8181:8080"
+    labels:
+      - "traefik.enable=true"
+      - "traefik.http.routers.outline-sync.rule=Host(`sync.domverse.de`)"
+      - "traefik.http.routers.outline-sync.entrypoints=https"
+      - "traefik.http.routers.outline-sync.tls=true"
+      - "traefik.http.routers.outline-sync.middlewares=secHeaders@file,middlewares-authentik-new@file"
+      - "traefik.http.services.outline-sync.loadbalancer.server.port=8080"
+
+networks:
+  domnet:
+    external: true