feat(stripe): simplify pricing to monthly/yearly and add conversion rates

BREAKING CHANGE: Quarterly pricing plans removed

• Remove quarterly pricing plans (EUR/MAD), keep only monthly and yearly
• Add ConversionRateService for EUR/MAD credit conversion management
• Store comprehensive metadata on Stripe products and prices
• Enhance CreateProductResponse with discount and yearly price details
• Enrich StripePriceResponse with metadata-based pricing information
• Refactor code with dedicated helper methods for better maintainability
• Add @Transactional support for product creation flow

Closes IAV-576