Complete all missing domain implementations in Client SDK v2

Add all 9 missing domains to Client SDK v2. For each domain, define Pydantic-based request/response DTOs under common/dto/manager//, implement a \*Client class under client/v2/domains/.py, and register it as a cached_property in BackendAIClientRegistry.

Missing domain list:
1. domain — Domain (tenant) CRUD
2. user — Admin-level user CRUD
3. keypair — KeyPair CRUD
4. image — Container image management
5. network — Network management
6. quota_scope — Storage quota management
7. system — System version info
8. acl — Permission listing
9. service_auto_scaling_rule — Auto-scaling rule CRUD

Each Story covers the DTO + v2 client implementation for its respective domain. Handler modifications are out of scope.

JIRA Issue: BA-4531