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

vitest.config.ts

@@ -9,8 +9,8 @@ export default defineConfig({
     setupFiles: ['./src/test/setup.ts'],
     // Ensure we're using the right environment
     environment: 'node',
-    // Only include test files
-    include: ['src/test/**/*.{test,spec}.{ts,js}'],
+    // Only include database integration tests
+    include: ['src/test/db-integration.test.ts'],
     // Configure coverage collection
     coverage: {
       provider: 'v8',