[CLEAN] Synthetic Benchmark PR #25465 - Changed member welcome email job to run based on config

[tomerqodo]

Benchmark PR TryGhost#25465

Type: Clean (correct implementation)

Original PR Title: Changed member welcome email job to run based on config
Original PR Description: ref https://linear.app/ghost/issue/NY-763

• Modifies the member welcome email job so it no longer references the welcomeEmails labs flag, and instead checks to ensure memberWelcomeEmailTestInbox is set in config
• This will allow us to stress test a higher volume of emails than we'd get by manually turning on the labs flag for a few sites
• By making sure the config exists before scheduling the job there won't be any unnecessary impact to self-hosted instances
• Doesn't send the email to any real members, just to an internal (to Ghost) test inbox
• The welcomeEmails lab flag itself isn't deleted yet - we'll likely use it for UI-related toggles in the future
  Original PR URL: Changed member welcome email job to run based on config TryGhost/Ghost#25465