Custom subscription intervals ("every N days")

Please add an interval multiplier to subscriptions so a plan can renew every
10 days or every 3 weeks, not only on the six fixed units available today
(daily, weekly, monthly, quarterly, every six months and yearly).

Use case: a couple of my clients sell physical consumables and are moving to
FluentCart to run subscriptions. For this kind of product the renewal cadence
is very important.

Example: consumers do not subscribe to "a month of dog food", they subscribe
to "a bag every 10 days" (I hope that makes sense 🙂). Someone buying almond
milk or coffee every 2 weeks has the same problem, one step down.

In both cases the closest interval does not match the customer's actual
consumption, and that mismatch may well be the main reason replenishment
subscriptions get cancelled. Right now the workaround is to pick the wrong
cadence and hope the customer tolerates it.

So my suggestion is an interval count alongside the existing period.

One of those clients is migrating from WooCommerce, where Subscriptions has
supported "every N period" for years (and the subscription apps on Shopify
expose custom day intervals too).

I know the six fixed units cover digital and membership products well, but
they do not cover physical replenishment - the category I'm working in, and
one with real growth potential. DoorDash and Amazon built much of their
business on exactly this model: the same customer, the same product, on a
rhythm they never have to think about.

That is why I'm asking for this feature.

Thanks Shahjahan Jewel and team for considering it 🙏