Rate limit users write endpoints

[brentthomas248]

Closes #1077

Summary

• add ThrottlerGuard coverage to UsersController
• throttle create, update, and delete user write endpoints to 5 requests per minute
• document 429 responses for throttled write endpoints
• add focused metadata coverage to keep read endpoints unthrottled

Tests

• npm test --prefix backend -- users.throttle.spec.ts --runInBand
• npm test --prefix backend -- users.controller.spec.ts users.throttle.spec.ts --runInBand
• npm test --prefix backend -- users-module --runInBand
• cd backend && npx --no-install eslint src/users-module/users.controller.ts src/users-module/users.controller.spec.ts src/users-module/users.throttle.spec.ts
• git diff --check -- backend/src/users-module/users.controller.ts backend/src/users-module/users.controller.spec.ts backend/src/users-module/users.throttle.spec.ts

Notes

• TypeScript compile is still blocked by pre-existing syntax errors in src/social-link/social-links.controller.spec.ts:293, unrelated to this users change.