Maintain Local Channels database

From @bumi :

We need to save all the required data in our DB and define single API calls to get all data.
Right now for example we do many calls to mempool triggered from the frontend to load the alias (not sure about rate limits there)

For example we can have a DB with channels that contains all the information from funding transaction to node alias and then have one GET /channels call to get this information from there. If I want information about the channel then I have a model that contains that data. right now if I want to get the node alias I would somehow have to go through this mempool API.

From Roland:

Also, we can save channel liquidity and other info that will be useful when events happen (like channel closures).

I would like to see an email notification or nostr DM when I get a channel closure:

"Your 500,000 sat channel with Olympus was closed. Reason: XYZ. You had 200,000 sats outgoing capacity when the channel was closed and these sats will be returned to your Alby Hub savings account within N blocks (about 2 weeks).

What to do next: ``"

Start by mapping the proposed GET /channels entry point and the frontend-triggered mempool calls described in the issue. Clarify which channel and liquidity data must be persisted, which API responses are required, and whether closure notifications are in scope; the work is done only when those requirements and their tests are defined.

Written by the indexing model from the issue text.