[Feature] Marketing permissions capture + Discord join prompt on sign-up

[iiamlewis]

Overview

When a player creates an account, two things must happen:

1. Explicit opt-in for marketing emails captured (GDPR-compliant)
2. Discord server join prompt shown

Scope

• Checkbox / toggle for marketing email opt-in during sign-up
• Opt-in status stored against user record
• Discord server invite shown on sign-up completion screen
• Mailchimp integration: opted-in users added to mailing list automatically

Acceptance criteria

• Marketing opt-in presented during sign-up (not buried)
• Opt-in status stored in DB against user
• Opted-in users synced to Mailchimp list
• Discord invite shown post sign-up
• GDPR-compliant — explicit opt-in only, no pre-checked boxes

Dependencies

• Depends on: OAuth / Google login
• Blocks: Retention email strategy, abandoned cart flow