Get currently logged-in customer ID

Use Cases:

• Customer portal initialization - determine who’s logged in
• Fetch customer-specific data (subscriptions, orders, settings)
• Validate customer session before displaying sensitive information
• Personalize customer portal UI with customer name/email

How it works:

• Customer must be logged in to their Shopify customer account
• Session is validated via Shopify App Proxy
• Returns Shopify customer ID (numeric)

Important Notes:

• This endpoint only works when called from the shop domain (via Shopify App Proxy)
• Customer must be logged in to Shopify
• Returns null if customer is not authenticated
• Do not use this for external API integrations - use External APIs with explicit customer ID instead

Authentication: Requires active Shopify customer session (browser-based)