back(test): specs validation env + DTO upload (48 -> 59 tests)

- env.validation.spec : config valide/coercion/défauts + rejets (var manquante,
  port hors plage, NODE_ENV inconnu)
- create-api-upload.dto.spec : name requis, emails @ValidateIf (vides OK,
  invalides rejetés), énum convention, MaxLength
- vitest-setup charge reflect-metadata (décorateurs class-validator en test)

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

back/vitest.config.ts

@@ -4,6 +4,7 @@ export default defineConfig({
   test: {
     globals: true,
     environment: 'node',
+    setupFiles: ['./vitest-setup.ts'],
     include: ['src/**/*.spec.ts'],
     coverage: {
       provider: 'v8',