fix: lnurl_encoding error was not handled

[dni]

when creating a withdraw link with an improperly configured proxy server a InvalidUrl error was thrown but not handled, resulting in returning a ValueError and an empty error message for clients.

• refactor to use helpers instead of putting logic in models
• lnurl_response was unused
• update poetry.lock

[motorina0]

Why take it out of the model?
It is a withdraw link specific functionality and it cannot be used without a withdraw link.

[dni]

i think it is better there, it took me a while to find the cause of this issue, it was not obvious for me that a model function had logic in it and was able to throw an exception. i think its makes it more obvious inside the helper and keeps the model simple.