Clickhouse Customer Users Destroy

Consolidated single/detail view for customer users.

Serves BOTH:

• Public API: /api/users/<customer_identifier>/ (URL path lookup)
• Dashboard: /clickhouse/customer-users/<id>/?customer_identifier=xxx (query param lookup)

Retrieves customer user from PostgreSQL (not ClickHouse) because:

1. Point retrieval is weak in ClickHouse
2. Update operations require PostgreSQL
3. All aggregation data is already returned in the list view

This view returns editable fields + real-time usage from Redis. Rate limited to protect against expensive point queries.

Uses SuperAdminMixin for queryset routing + object-level ownership. No manual org injection needed - SuperAdminMixin handles patch() automatically.