Register a new airline partner for direct data submission.

Admin-only endpoint. Creates entry in airline_partners table and generates a unique API key for subsequent submissions.

Input JSON:

• airline_name (str, required): Full airline name
• contact_email (str, required): Partner contact email
• program_slug (str, required): Loyalty program slug (must match one of 26 programs)
• website_url (str, optional): Airline's website

Returns:

• api_key (str): afd_partner_<32 hex chars> — store securely
• partner_id (str): UUID for this partner
• program_slug (str): Normalized program slug
• created_at (str): ISO timestamp