implement LNURL auth

[louneskmt]

Implement LNURL auth login considering that every account can link multiple wallets. That means it cannot be a single linking_public_key column on our user table. Instead, we likely need a table specific to LNURL public keys that does the link between user and keys.

Expose the appropriate API routes.

For the front, plug LNURL to the register and login modals. No need for implementing multiple wallets in the settings yet, we'll see later.