feat: add Saved Filters CRUD API

- Create lib/api/saved-filters.ts with GET, POST, PATCH, DELETE endpoints
- All endpoints require authentication via requireAuth guard
- Users can only access their own saved filters
- Register savedFilterRoutes in lib/api/index.ts

lib/api/index.ts

@@ -20,3 +20,4 @@ export { compareRoutes } from "./compare"
 export { playabilityRoutes } from "./playability"
 export { steamReviewRoutes } from "./steam-reviews"
 export { communitySuggestionRoutes } from "./community-suggestions"
+export { savedFilterRoutes } from "./saved-filters"