🎨 Added warning and filter for members with multiple active subscriptions

[kevinansfield]

closes https://linear.app/ghost/issue/BER-3721/

Members with multiple active Stripe subscriptions are usually paying twice without realising it, and until now there was no easy way for site owners to spot them. The Members list now shows a banner whenever any members have multiple active subscriptions, with a link to filter down to the affected members so the duplicates can be resolved. How it works is documented in the duplicate subscription warning help doc.

[coderabbitai]

Walkthrough

This PR deprecates the multipleSubsFilter feature flag by removing it from the GA features registry and eliminating the corresponding multipleSubsFilterEnabled prop from the Members page component. The MultipleActiveSubscriptionsBanner rendering condition is simplified to depend solely on hasStripeEnabled, removing the additional feature flag gate.

Possibly related PRs

• TryGhost/Ghost#28232: Adds the MultipleActiveSubscriptionsBanner with multipleSubsFilter lab flag gating and the filter UI behavior that this PR deprecates.
• TryGhost/Ghost#28507: Fixes the backend query for computing active Stripe customer counts, which affects the data displayed by the multiple subscriptions banner being ungated in this PR.

Suggested reviewers

• 9larsons
• jonatansberg

🚥 Pre-merge checks | ✅ 3 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Title check	⚠️ Warning	The title describes adding a warning and filter for members with multiple subscriptions, but the PR actually removes the feature flag and associated conditionals to complete a rollout—not add new functionality.	Update the title to reflect the actual change: something like 'Remove multipleSubsFilter GA flag to complete rollout' or 'Graduate multiple subscriptions warning from experimental to default'.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The pull request description accurately describes the changeset, explaining the removal of the multipleSubsFilter GA flag and clarifying that the feature is now fully rolled out.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch duplicate-subscription-warning

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