test: cover feature flags controller

[brentthomas248]

Summary

• adds controller unit coverage for the feature flags endpoint
• verifies FeatureFlagsController delegates to the mocked FeatureFlagsService
• verifies the returned snapshot is passed through unchanged

Closes #1078

Verification

• npm test --prefix backend -- feature-flags.controller.spec.ts
• npm test --prefix backend -- feature-flags.service.spec.ts feature-flags.controller.spec.ts
• git diff --check HEAD~1..HEAD -- backend/src/feature-flags/feature-flags.controller.spec.ts
• cd backend && npx --no-install eslint src/feature-flags/feature-flags.controller.spec.ts