Discord Integration

[JoaquinBN]

Discord integration for the Community section. Includes OAuth for account linking, XP sync from Discord bot, and role sync.

• Implement Discord OAuth flow (part of onboarding?)
• Link Discord account to user profile
• Design Discord XP sync mechanism (API or bot integration — TBD)
• Document XP to GP conversion approach
• Sync Discord roles to platform roles
• Display Discord XP on profile