EnsureCurrentAsync now takes viewerId (was Viewer), so it works with
LoadController's AsNoTracking-loaded detached viewers.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Reads existing state from DB on each call (don't trust navigation
property — caller may pass it stale or double-tracked). Adds via DbSet
only, not via navigation property, to avoid EF double-tracking.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>