Upgrade to Pydantic 2.x -- blocked by lnurl

[AngusP]

• Requires upgrade in lnurl library Upgrading to Pydantic 2 lnbits/lnurl#18

With only a single user supported, all the methods on PhoenixdLNURLSettings can be changed to

class PhoenixdLNURLSettings(BaseSettings):
    ...

    @pydantic.computed_field
    @functools.cached_property
    def example(self) -> Any:
        ...

Pydantic 2 docs: Computed Fields

So that their return values are computed once then cached for the lifetime of the program.

The negative impact of not caching is probably minimal, but it still feels a bit dumb to e.g. regenerate the exact same QR code and json dump string for every request