Resolve linter issues, add unit tests, adjust test coverage

qr-api/vitest.config.ts

@@ -4,5 +4,17 @@ export default defineConfig({
     test: {
         globals: false,
         environment: 'node',
+        coverage: {
+            provider: 'v8',
+            reporter: ['text', 'lcov'],
+            include: ['src/**/*.ts'],
+            exclude: ['src/**/*.test.ts', 'src/index.ts'],
+            thresholds: {
+                lines: 90,
+                functions: 100,
+                branches: 72,
+                statements: 90,
+            },
+        },
     },
 });