feat: add refferal feature

lib/api/referral.ts

@@ -0,0 +1,25 @@
+import { apiClient } from './client';
+import { PendingReferralRewardsResponse, FulfillReferralRewardResponse } from '@/types/referral';
+
+export const referralApi = {
+    // List users who reached the invite target and have not been granted the free month yet
+    getPendingRewards: async (token: string): Promise<PendingReferralRewardsResponse> => {
+        return apiClient.get<PendingReferralRewardsResponse>('/admin/referral-rewards', token);
+    },
+
+    // Grant the free month to a user. product_id is optional: omit it to
+    // auto-grant the meditation package's monthly product.
+    fulfillReward: async (
+        userId: number,
+        token: string,
+        productId?: number
+    ): Promise<FulfillReferralRewardResponse> => {
+        const data = productId ? { product_id: productId } : {};
+
+        return apiClient.post<FulfillReferralRewardResponse>(
+            `/admin/referral-rewards/${userId}/fulfill`,
+            data,
+            token
+        );
+    }
+};