fix(gmail): paywall additional inboxes, keep first inbox free

[gbirman]

Restores the professional-subscription gate on /link/gmail that #4277 removed, but only paywalls when the user already has an inbox — so free users can still link their first inbox. Supersedes #4277's blanket removal with the proper "first inbox free" gating, and adds a unit test safeguarding the decision.

Macro task: https://macro.com/app/task/019ef4c3-ac39-79b1-83ff-e0b19c520cd8

[macro-application]

multi-inbox account linking - check if user already has an account linked before paywall

[coderabbitai]

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📝 Walkthrough

Walkthrough

The Gmail inbox-link initialization handler (init_gmail_link_handler) is updated to enforce a paywall for linking additional inboxes. The handler now extracts user permissions, computes whether the user has professional features, and queries the database for existing inbox ownership when the user is not pro. A new private helper, requires_professional_subscription, encapsulates the paywall decision—returning true only when the user lacks professional features and already owns an inbox—and returns PaymentRequired in that case. Imports are consolidated, a #[allow(dead_code)] attribute is removed, and a test module with three unit tests covering all input combinations is added.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title follows conventional commits format (fix:), is 61 characters (under 72), and clearly describes the main change: implementing a paywall for additional Gmail inbox links while keeping the first inbox free.
Description check	✅ Passed	The description is directly related to the changeset, explaining the restoration of professional-subscription gating with conditional logic for first inbox free, and references the superseded PR and added tests.
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.

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