chore(BA-5822): drop legacy AppConfig SDK + CLI + DTO surfaces

Extends the legacy-AppConfig drop to the client side:

- `client/v2/domains_v2/app_config.py` — `V2AppConfigClient` with
  `upsert_domain_config` / `delete_user_config` / etc. is removed
  alongside its `v2_registry` `@cached_property` wiring.
- `client/cli/v2/app_config/` — legacy `./bai app-config get-domain`,
  `upsert-user`, etc. are removed from the CLI tree.
- `common/dto/manager/v2/app_config/` — legacy DTO module
  (`UpsertDomainConfigInput`, `DeleteUserConfigPayload`, ...) is
  deleted.

Per #11295 review: BEP-1052's new SDK / CLI / DTO surfaces will land
in their own PRs and should not also have to repeal the legacy
versions there.

Author: jopemachine

src/ai/backend/client/cli/v2/__init__.py

@@ -251,11 +251,6 @@ def notification() -> None:
     """Notification commands."""
 
 
-@v2.group(cls=LazyGroup, import_name="ai.backend.client.cli.v2.app_config:app_config")
-def app_config() -> None:
-    """App config commands."""
-
-
 @v2.group(
     cls=LazyGroup,
     import_name="ai.backend.client.cli.v2.prometheus_query_preset:prometheus_query_preset",

src/ai/backend/client/cli/v2/app_config/__init__.py

@@ -16,7 +16,6 @@
 
 if TYPE_CHECKING:
     from .domains_v2.agent import V2AgentClient
-    from .domains_v2.app_config import V2AppConfigClient
     from .domains_v2.artifact import V2ArtifactClient
     from .domains_v2.artifact_registry import V2ArtifactRegistryClient
     from .domains_v2.audit_log import V2AuditLogClient
@@ -89,12 +88,6 @@ def agent(self) -> V2AgentClient:
 
         return V2AgentClient(self._client)
 
-    @cached_property
-    def app_config(self) -> V2AppConfigClient:
-        from .domains_v2.app_config import V2AppConfigClient
-
-        return V2AppConfigClient(self._client)
-
     @cached_property
     def artifact(self) -> V2ArtifactClient:
         from .domains_v2.artifact import V2ArtifactClient