Create coordinator_duplicate_queue database view

Because Supabase PostgREST exposes views as REST endpoints automatically, naming the view coordinator_duplicate_queue makes it queryable from Dart without additional configuration. Use auth.uid() inside the view definition (or in a row-level security policy on the view) to resolve the coordinator's chapter memberships dynamically. If PostgreSQL does not support auth.uid() directly in view definitions in your Supabase version, use a security barrier view (WITH (security_barrier = true)) and attach an RLS policy instead. The candidate_count column can be a subquery or a lateral join counting related duplicate candidate records — keep it simple and avoid window functions if they prevent index usage.

Consider adding a created_at ordering so the coordinator sees the oldest unresolved duplicates first, prioritising stale items in the queue.

SQL test cases: (1) Coordinator from Chapter A queries view — receives only Chapter A unresolved duplicates; (2) Coordinator from Chapter B queries same view — receives only Chapter B items; (3) After marking an activity as duplicate_reviewed=true, that activity no longer appears in the view; (4) Empty chapter returns empty set without error; (5) View is accessible via Dart client using supabase.from('coordinator_duplicate_queue').select() without 403/404. Test RLS isolation by creating two test coordinator users assigned to different chapters in the seed data. Run rollback script and confirm view is dropped, then re-run migration and confirm view is recreated correctly.