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/transactions/index.ts

@@ -69,7 +69,13 @@ export const POST: APIRoute = async ({ request }) => {
       tags: transaction.tags,
     });
 
-    return new Response(JSON.stringify(newTransaction), {
+    // Convert Decimal to number for response
+    const response = {
+      ...newTransaction,
+      amount: Number(newTransaction.amount),
+    };
+
+    return new Response(JSON.stringify(response), {
       status: 201,
       headers: { 'Content-Type': 'application/json' },
     });