Adjusted return type, since async is no longer needed here (thanks to db relations)

ref #56

src/models/entities/Donation.ts

@@ -31,5 +31,5 @@ export abstract class Donation {
    * The donation's amount in cents (or whatever your currency's smallest unit is.).
    * The exact implementation may differ for each type of donation.
    */
-  abstract amount: number | Promise<number>;
+  abstract amount: number;
 }