Get customer portal link

Create a Stripe customer portal session link for the authenticated user.

Use this endpoint to send users into self-serve billing management.

Behavior:

• Reuses existing Stripe customer link when present.
• If missing, creates a Stripe customer record using account/profile + address data.
• Returns a fresh customer portal URL for immediate redirect.

Failure mode:

• Returns 404 if the upstream user account cannot be resolved.