Queue new plugin notifications via a dedicated job

[simonhamp]

Summary

• Moves the NewPluginAvailable notification fan-out from Plugin::approve() into a new SendNewPluginNotifications queued job
• Previously, approving a plugin iterated through all opted-in users synchronously during the HTTP request, queuing individual notifications one-by-one — this blocked the admin UI
• Now a single job is dispatched, and the fan-out happens on the queue worker

Changes

• New: app/Jobs/SendNewPluginNotifications.php — queued job that queries opted-in users and sends NewPluginAvailable notifications
• Updated: Plugin::approve() — dispatches the job instead of inline Notification::send()
• Updated: NewPluginAvailableTest — integration tests now assert the job is dispatched via Bus::fake()
• New: SendNewPluginNotificationsTest — dedicated tests for the job's notification logic

Test plan

• Job sends notifications to opted-in users
• Job excludes the plugin author
• Job is dispatched on first approval
• Job is not dispatched on re-approval
• All existing notification tests still pass

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