feat: add Steam Reviews display API with caching

- Create lib/api/steam-reviews.ts with in-memory cache (1hr TTL)
- Proxy endpoint for fetching Steam reviews by game ID
- Supports offset/limit/language query params
- Registers route in API index and app router

lib/api/index.ts

@@ -18,3 +18,4 @@ export { steamgridProxyRoutes } from "./steamgrid-proxy"
 export { gamesManualRoutes } from "./games-manual"
 export { compareRoutes } from "./compare"
 export { playabilityRoutes } from "./playability"
+export { steamReviewRoutes } from "./steam-reviews"