Implement occasional maintenance task to remove old non-critical data (request and response events)

[rolznz]

We have DB constraints (currently 1GB) and the NWC database will grow over time, especially due to storing NIP-47 requests and responses. Ones older than 3 months could be occasionally deleted.

[rolznz]

Because this requires a migration (and might be easier to just drop and create a new table since our app is still unpublished) we would also like to change the table to transactions and include incoming payments as well (as per #105) and make sure we have enough info to do better budgets (#122) plus this requires more planning to make sure we have everything transaction-related we need for Alby Hub.

For now we are postponing this (but is still important)