## Problem
The Discord **Plan changed** notification only showed the *new* plan
value in the `Changed` field, so it was impossible to tell what actually
changed:
```
Changed
Plan: €3.99 / month
```
Was it a price change? An interval change? From which plan? No way to
know.
## Fix
Compute the previous plan from Stripe's `previous_attributes` and render
it as `old → new`, matching the existing status-change behaviour:
```
Changed
Plan: €9.99 / month → €3.99 / month
```
`planLabel()` was generalised to read both the modern
`items.data[].price` shape and the legacy top-level `plan` shape Stripe
still includes in the diff, so the same helper works on both the current
object and the `previous_attributes` diff. Falls back to the old `Plan:
X` output when no previous value is detectable.
## Test
Added a feature test asserting the `old → new` output for a
`customer.subscription.updated` plan change. All 11 tests in the file
pass.
3972007844