feat(auth): signup flow with org creation/joining (domain matching)

[PierreBrisorgueil]

Description

Modify the signup flow to handle organization creation or joining based on config and email domain.

Details

When organizations enabled + domainMatching:

1. User signs up with email
2. Extract domain from email (e.g. @acme.com)
3. Check if an organization exists with that domain
   • Yes → create membership (role: member) on that organization
   • No → require organization creation (name, slug) → membership (role: owner)

When organizations enabled + no domainMatching:

1. User signs up
2. Organization creation step required (name, slug)

When organizations disabled:

1. User signs up
2. Organization auto-created in background (invisible)
3. Membership (role: owner) created silently

Response

Signup response includes the organization info and abilities so the frontend can adapt.

Acceptance criteria

• Domain matching finds existing organizations
• New organization created when no match
• Silent organization creation when organizations disabled
• Signup response includes organization + abilities
• Integration tests for all 3 scenarios