refactor(model): adopt flattened protobuf packages and AIP-compliant resource names

[pinglin]

Because

• The protobuf packages have been flattened (e.g., model/model/v1alpha → model/v1alpha, core/mgmt/v1beta → mgmt/v1beta)
• API requests now use AIP-standard Parent field instead of NamespaceId for resource hierarchy
• AIP standard requires auto-generated resource IDs and slug-based human-friendly URLs
• Usage reporting has been moved to a centralized service

This commit

• Updates all protobuf imports to use flattened package paths:
  • protogen-go/model/model/v1alpha → protogen-go/model/v1alpha
  • protogen-go/artifact/artifact/v1alpha → protogen-go/artifact/v1alpha
  • protogen-go/core/mgmt/v1beta → protogen-go/mgmt/v1beta
• Updates handler methods to parse Parent field instead of NamespaceId
• Updates mgmt API calls to use AIP-compliant request formats (e.g., Permalink instead of UserUid)
• Adds database migrations for slug and aliases columns on model table
• Adds GetNamespaceModelUIDByID method for internal UID lookups
• Removes pkg/usage/usage.go (~300 lines) - usage reporting moved to centralized service
• Removes pkg/db/pagination.go - pagination utilities moved to shared library
• Updates integration test proto files to match new flattened structure
• Removes deprecated nested proto files from integration tests