Unified endpoint to create invitations for users OR admins based on role.

Authorization:

• Only admin users can create invitations (for both admins and regular users)
• Non-admin users will receive 403 Forbidden

For Admin Invitations (role="admin"):

• Creates Firebase user with SSO authentication
• Sends activation email with 24-hour expiry token
• One membership per organization. Inviting an email that belongs to another organization is rejected unless ALLOW_MULTI_ORG_ADMINS is on
• An email that is already an active admin here is reported per email as already_a_member; its membership is left untouched
• Uses business_admins collection

For User Invitations (role != "admin"):

• Validates emails exist in Firestore
• Checks users are not already in any business
• Creates invitation and sends email
• Uses invitations collection

Request Body:

• user_emails: List of email addresses
• role: "admin" | "user"
• invitation_expiry: Days until expiry (default: 7 for users, 24 hours for admins)