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>

api/ddns_manager.py

@@ -344,7 +344,9 @@ class DDNSManager(BaseServiceManager):
             return None
 
         if provider_name == 'pic_ngo':
-            api_base = ddns_cfg.get('api_base_url')
+            # Env var takes priority so deployments can switch URLs without re-registering
+            _env_url = os.environ.get('DDNS_URL', '').replace('/api/v1', '').rstrip('/')
+            api_base = _env_url or ddns_cfg.get('api_base_url')
             totp_secret = ddns_cfg.get('totp_secret') or os.environ.get('DDNS_TOTP_SECRET', '')
             return PicNgoDDNS(api_base_url=api_base, totp_secret=totp_secret)