Complete ProviderPricingConfig implementation (18 TODOs)

backend/src/models/ProviderPricingConfig.js has 18 TODOs covering: pricing API integration (currently static defaults), provider sync (no webhook/cron refresh), schema improvements (missing supportedFeatures, rateLimits, deprecationDate fields), caching strategy (in-memory only), and custom provider support (limited to OpenAI-compatible).