Implement Deduplication Queue Service

Implement using a repository pattern: DeduplicationQueueService depends on DuplicateQueueRepository (abstract interface) for testability. Use Supabase Realtime `.stream()` builder for queue subscriptions rather than `.on()` to get automatic RLS enforcement on the subscription. For bulk dismiss, use Supabase `rpc()` to call a Postgres function that performs the batch update atomically — do not loop individual updates from the client. Audit events should be inserted in the same transaction as the status update using a Postgres trigger or RPC.

Model audit entries using the existing claim_event pattern (actor_id, actor_role, from_status, to_status). Use Dart's sealed classes or a Result pattern for error propagation. Register the service via Riverpod Provider so it is scoped to the coordinator session and disposed on logout.

Unit tests: test fetchPendingItems with mocked DuplicateQueueRepository for empty queue, single item, and 50+ item scenarios. Test markAsReviewed emits correct PROCEED_WITH_DUPLICATE audit event shape. Test bulkDismiss rolls back on partial failure using mock that throws on item N. Integration tests: verify RLS prevents cross-organization queue access using two test organizations.

Verify Realtime subscription delivers updates within expected latency. Test audit log entries are correctly written to bufdir_export_audit_log with all required fields. Achieve minimum 85% line coverage on service class.