A B2B SaaS company is designing a new subscription billing platform on Google Cloud. The system processes invoicing and payment reconciliation where every monetary transaction must be strongly consistent, and financial reports frequently join billing data with account and usage tables using complex multi-table SQL queries. The business projects growth from thousands to millions of customer accounts across North America and Europe within three years, and requires horizontal scalability without sharding management or downtime for maintenance. Which storage service best meets these requirements?