REMNAWAVE REVERSE PROXY

🔧 Docker Images Fixes
Pinned Docker image versions to improve stability:

remnawave/backend: from latest to 1.6.16
remnawave/node: from latest to 1.6.4

🛡️ Security and Stability Improvements
Added container existence checks before performing template operations:

Check for remnawave container in the function manage_template_upload
Check for remnawave-subscription-page container in the function manage_sub_page_upload

If container is missing, an appropriate error message is shown