feat: Support yearly billing

[eseidel]

One customer has asked for yearly billing (I suspect others would want it to), specifically:

"I am not asking for a discount, but just for having a single invoice every year. It is a request from our accountant"

I think yearly billing is pretty common, but I'm not sure all the details, especially how it relates to usage-based billing like ours and potential overages. Would overages be billed yearly as well? Or just pre-payed and topped-up on some credits system?

I'm also happy to give discounts based on yearly payment cycles, my bigger questions are just what the right technical implementation here is for customers.

Based on that one customer's request it sounds like what they're seeking is simplicity (which makes sense). The question is how to make this simple.

Suggestions welcome!

[eseidel]

Customers have also asked for 3mo or 6mo billing links. We've had several requests from India in particular where my understanding is that reoccurring credit card charges are harder.

[eseidel]

We've now successfully done this once and are able to support multi-month pre-paid billing one a customer-by-customer basis. Leaving this bug open because we should make this self-serve over time.