Change Stripe Subscription handling to manage unpaid state

### Description

This is a follow-up to https://github.com/flowforge/flowforge/issues/1878, raising as its own task for planning purposes.

We currently configure Stripe to cancel a user's subscription after a number of failed payments. That leaves us in the position that if they then setup a new subscription for the team, all existing debts are forgotten.

Stripe puts the subscription into the `unpaid` state before this happens - something we don't track.

We should:

1. modify our subscription handling to *not* cancel the subscription when it goes unpaid.
2. track the unpaid state and take appropriate action.
- That could be to prevent the user from doing anything billable in the platform until they pay their bills.

### Epic/Story

_No response_

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate