IAV-1212: add credits field to subscription prices with performance optimizations
- Add BigDecimal credits field to StripePriceResponse
- Add computeCredits() and computeAnnualCredits() public methods in StripeProductHelper
- Fix annual credits to use full_price_without_discount_cents metadata (not discounted amount)
- Fix NPE on null conversionRate in compute methods (return BigDecimal.ZERO)
- Fix ArithmeticException on BigDecimal.divide by adding explicit scale/rounding
- Fix missing conversionRateEur/Mad in updateProductWithPlans response
- Optimize getSubscriptionProducts: single DB call instead of N x2 queries
- Optimize findMostPopularProductId: single Stripe API call with in-memory groupingBy
- Deduplicate updateConversionRates (delegates to createConversionRates)
- Centralize computeCredits/computeAnnualCredits in helper (remove duplicates from service)
Closes IAV-1212