back(test): coverage vitest (@vitest/coverage-v8)

Ajoute le provider de coverage + section coverage dans vitest.config (reporter
text/html/lcov, exclusions modules/entities/dto/main) et un script test:cov.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

back/vitest.config.ts

@@ -5,5 +5,18 @@ export default defineConfig({
     globals: true,
     environment: 'node',
     include: ['src/**/*.spec.ts'],
+    coverage: {
+      provider: 'v8',
+      reportsDirectory: './coverage',
+      reporter: ['text', 'html', 'lcov'],
+      include: ['src/**/*.ts'],
+      exclude: [
+        'src/**/*.spec.ts',
+        'src/**/*.module.ts',
+        'src/**/*.entity.ts',
+        'src/main.ts',
+        'src/**/*.dto.ts',
+      ],
+    },
   },
 });