[FEAT] Add Earnings Export Scheduled Email

[Emmzyemms]

Description:
Creators want to automatically receive their monthly earnings report by email for tax purposes.

Acceptance Criteria:

• Add a "Monthly Report Email" toggle in Settings → Notifications
• POST /api/earnings/schedule-report enables/disables the scheduled export
• A cron job (Vercel Cron or external) fires on the 1st of each month
• Cron calls POST /api/cron/earnings-reports with an admin Bearer token
• Route generates and emails CSV exports to all users with the toggle enabled
• Email uses a branded HTML template with the earnings summary and CSV attachment
• Opt-in email preference is stored in the database

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