Api User Update Retrieve

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.