Self-registration with org discovery

[slifty]

Description

Allow users to register without an invitation and discover/request to join organizations.

Requirements

• User registers without an invitation code (standard email+password registration)
• After registration, user lands on a dashboard with options:
  • Create a new organization
  • Enter an invitation code
  • Browse/search organizations (if enabled)
• Organization discovery:
  • Org setting: discoverable (boolean) — whether the org appears in search
  • Search organizations by name
  • View org profile (name, description, logo, website)
• Request to join an organization:
  • User submits a join request
  • Org admin receives notification and can approve/deny
  • On approval, user is added as org member
• Org admin: view pending join requests, approve/deny with optional message

Notes

This is a lower-priority onboarding path. Most users will join via invitation codes. But for public organizations that want to allow open membership, this flow is needed.