Assign Organizations Automatically by Domain

Enable automatic organization assignment when creating users. Match user email domain against organization domains and auto-assign to matching organization. Support manual addition of users with non-matching email domains.

Note:

• On user creation, extract domain from email address
• Query organizations for matching domain (from TBD-07)
• Auto-assign user to organization if domain matches
• UI should show which organization will be assigned (if any) before creation
• Allow admin override: manually assign to different org or no org
• Support adding users with email addresses that don't match any org domain
• Domain matching: case-insensitive, stored lowercased
• Domain uniqueness: only one org can claim a domain as default
• Domain changes: existing memberships remain, new assignments use current domain config