feat(server): add unified entity manager with duplicate detection

- Remove author insights and duplicate suggestions panels, composables, DTOs, and all backend support
- Add genre and tag duplicate detection, merge, and dismiss with server-side paginated scan results
- Consolidate per-entity-type tools into a single entity-manager module backed by the Strategy pattern
- Support browse, merge, rename, delete, split, and dismiss across all entity types (author, genre, tag, narrator, publisher, language, series)
- Replace old tools module with unified entity-manager controller, service, and repository; add 85+ unit tests

server/src/app.module.ts

@@ -54,6 +54,7 @@ import { AuditModule } from './modules/audit/audit.module';
 import { MigrationModule } from './modules/migration/migration.module';
 import { NotificationModule } from './modules/notification/notification.module';
 import { SeriesModule } from './modules/series/series.module';
+import { EntityManagerModule } from './modules/entity-manager/entity-manager.module';
 
 @Module({
   imports: [
@@ -116,6 +117,7 @@ import { SeriesModule } from './modules/series/series.module';
     MigrationModule,
     NotificationModule,
     SeriesModule,
+    EntityManagerModule,
   ],
   providers: [
     { provide: APP_GUARD, useClass: ThrottlerGuard },