docs: Add comprehensive email requirements analysis

Commit notes

Analyzed the entire Samplestep codebase to identify all email types needed:

Authentication & Account: - Welcome, email verification, password reset, account deletion

Subscription & Billing: - Confirmation, receipts, payment failures, cancellation, expiry notices

Storage Alerts: - 80% warning, 100% limit reached (schema already supports)

Marketplace: - Purchase receipts (buyer), sale notifications (creator), refunds

Payouts: - Request confirmation, completion, failure notifications

Workspace Collaboration: - Invitations, acceptance/decline, member removal, role changes

Share Links (user-configurable preferences exist): - Access, download, comment, expiring, upload received

Upload Links: - Created, uploads received, review notifications

Creator Onboarding: - Mollie Connect status updates

System: - Weekly/monthly digests, announcements, security alerts

Includes priority matrix and technical implementation recommendations.

Co-authored-by: armin.naimi <[email redacted]>