Billing page shows incomplete upcoming charges for teams with annual plan and monthly add-ons

## Summary

When a team has a yearly or monthly base subscription and also pays monthly for extra hosted/remote instances or devices, the team billing page Next Payment shows the first pending invoice. This hides follow-up monthly bills and gives a wrong picture of upcoming charges.

## Why

- with the current (outdated) Stripe SDK we can only fetch a single upcoming billable invoice.
- not an issue when there is one subscription per team, since we show the next team billable event (could be 1 year out)
- Incomplete when monthly add-ons exist, since we only show the first immediate monthly bill (~1 month out) and not the follow-ups
- Yearly billing for add-ons is not available yet. See follow-up: Annual billing for additional hosted/remote instances (#5913)

## References

- required https://github.com/FlowFuse/flowfuse/issues/5910
- follow up of: https://github.com/FlowFuse/flowfuse/pull/5896
- part of https://github.com/FlowFuse/flowfuse/issues/4318

Begin with required issue #5910 and the follow-up PR #5896, then trace the billing page's Next Payment entry point. Confirm how upcoming invoices are fetched and represented for base subscriptions with monthly add-ons. Done means the page presents the relevant follow-up monthly charges rather than only the first pending invoice.

Written by the indexing model from the issue text.