Allow to set max base fee for withdrawals

[ekzyis]

Is your feature request related to a problem? Please describe.

I configured 1% as the max fee for autowithdrawals to my phoenixd node. I noticed that withdrawals smaller than 100 sats don't seem to work with phoenixd. I see no route found errors in the wallet logs. I strongly suspect this is because the ACINQ node takes a base fee of 1 sat for receives which means that 1% of anything smaller than 100 sats is not enough.

Since the channel between SN and ACINQ is not public, I tested this by creating a 1 sat invoice from phoenixd and paid it using my sats on SN and noticed that indeed, there was 1 sat as a routing fee:

When we switch to non-custodial, this means that I would receive fee credits for any zap that is smaller than 100 sats.

Describe the solution you'd like

I am fine with paying 1 sat to receive 1 sat even if it means that I pay 50% as routing fees. That's just how lightning base fees work. I think we should allow to set a base fee and then use whatever is higher as the max fee: max_fee = max(max_percentage_fee, max_base_fee)

Describe alternatives you've considered

• don't care about small zaps becoming fee credits in the future if I continue to use phoenixd
• run node with private channel to SN again

Additional context

n/a