How should we trust you with the backend?
You could just open a backdoor to all the bitcoin deposits for yourself. Not saying that you will but this is a possibility.
Is there a trustless way to do this?
I provide a way to integrate Blockchain address using the user's API, not my own.
If you need any more help, let me know.
EDIT: For better clarifications, I will not have the access to call a request unless the user provides me the keys. SO i won't be able to process any transaction. I am a web developer, not a hacker lmao.