[Feature request] Add NWC backend

Will be good to have also the option to use another NWC source as a backend for Alby Hub (like LND, LDK, cashu).
Example: I run a Rizful node that provide a NWC connection.
I could run easily at home on a PC an Alby Hub and connect it to that Rizful node. Then I can do more things with that Alby Hub.

Start by reviewing how Alby Hub currently integrates LND, LDK, and cashu backends, then examine the NWC connection provided by a Rizful node. Define the backend behavior and integration scope needed to connect an external NWC source and support the additional Alby Hub functionality described.

Written by the indexing model from the issue text.