fix: DDNS URL env var takes priority; switch default to HTTPS

- ddns_manager: DDNS_URL env var overrides stored api_base_url so
  existing cells pick up the new HTTPS endpoint without re-registering
- docker-compose.yml: default DDNS_URL now points to https://ddns.pic.ngo
- setup_manager.py: add rstrip('/') before replacing /api/v1 to handle
  URLs with or without trailing slash

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

docker-compose.yml

@@ -205,7 +205,7 @@ services:
     ports:
       - "127.0.0.1:${API_PORT:-3000}:3000"
     environment:
-      - DDNS_URL=${DDNS_URL:-http://ddns.pic.ngo:8080/api/v1}
+      - DDNS_URL=${DDNS_URL:-https://ddns.pic.ngo/api/v1}
       - DDNS_TOTP_SECRET=${DDNS_TOTP_SECRET:-S6UMA464YIKM74QHXWL5WELDIO3HFZ6K}
     volumes:
       - ./data/api:/app/data