Fix EconomicEventItem type: add missing period field, make comparison nullable (#14)

- Add `period: string | null` — API returns period (e.g., "May", "Q1") but
  the type definition was missing it entirely
- Change `comparison: string` → `string | null` — API returns null when no
  comparison type applies

Author: alexpipipi

src/types.ts

@@ -539,7 +539,8 @@ export interface EconomicEventsParams extends DateRange, Pagination {
 
 export interface EconomicEventItem {
   type: string;
-  comparison: string;
+  comparison: string | null;
+  period: string | null;
   country: string;
   date: string;
   actual?: number | null;