Send payment confirmation emails to multiple payments in batch using Resend's batch API.

This endpoint efficiently sends payment confirmation emails for multiple payments at once, respecting Resend's rate limits and batch size constraints (up to 100 emails per batch).

Args: payment_ids: List of payment IDs to send confirmation emails for user_id: ID of the authenticated user (from auth middleware) org_id: ID of the user's organization (from auth middleware)

Returns: dict: Detailed results including sent_count, failed_count, skipped_count, and per-payment results

Raises: HTTPException: 401 if unauthorized, 400 if invalid input, 500 if server error