Replace in-memory store with database persistence

- Remove in-memory store and related tests
- Add Decimal to number conversion in API responses
- Update integration tests to handle Prisma Decimal type
- Fix test configuration to only run db-integration tests

src/pages/api/accounts/[id]/transactions/index.ts

@@ -5,7 +5,13 @@ export const GET: APIRoute = async ({ params }) => {
   try {
     const accountTransactions = await transactionService.getByAccountId(params.id as string);
 
-    return new Response(JSON.stringify(accountTransactions), {
+    // Convert Decimal to number for each transaction in response
+    const response = accountTransactions.map((transaction) => ({
+      ...transaction,
+      amount: Number(transaction.amount),
+    }));
+
+    return new Response(JSON.stringify(response), {
       status: 200,
       headers: {
         'Content-Type': 'application/json',