Add billing catalog control plane

[kaustubhkagrawal]

Summary

• add defineBillingCatalog and better-auth-openmeter-plugin/catalog
• validate app-owned meters, features, plans, prices, and provider ids
• compile catalog plans to OpenMeter entitlement plans
• compile provider-neutral product/price setup data for Stripe, Polar, Razorpay, Creem, Dodo, Autumn, and custom providers
• let the billing adapter map event.plan through a catalog or run in entitlementMode: "none" for managed catalog/subscription systems
• document the catalog setup workflow and bump the package to 0.3.0

Verification

• npm run typecheck
• npm test
• npm run build && npm pack --dry-run
• import smoke check for ./dist/catalog/index.js, ./dist/adapters/billing.js, and root ./dist/index.js