feat: implement Limits API (/v3/limits, /v3/limits-model, unified-limits)

[gtema]

Parent issue: #938

Implement the Limits API endpoints currently missing in the Rust Keystone.

Endpoints to implement

Endpoint	Actions
/v3/limits	GET (list, with include_limits flag), GET {id} (show)
/v3/limits-model	GET (model discovery)
/v3/unified-limits	GET (if present)

Reference

Python Keystone limits API: openstack/keystone - limits module

Tempest limits tests: tempest.api.identity.v3.limits.*

Notes

• Review Python Keystone for reference behavior
• Policy enforcement via OPA Rego in policy/ directory
• Follow existing CRUD patterns (backend.rs convention: get_, list_)
• Unit tests: valid auth + positive/negative policy, invalid auth (per AGENTS.md)