Create Billing Portal Session

Create a Stripe Billing Portal session for a team.

Allows users with billing permissions to manage their subscription, payment methods, and invoices through Stripe's hosted portal.

Args: team_id: Team UUID. return_url: URL to redirect the user back to after the portal. auth: Authenticated user context.

Returns: BillingPortalResponse with the portal URL.

Raises: HTTPException: 403 if not a member or lacks permission, 400 if return_url is untrusted or no Stripe customer exists.