docs(server): say why the admin domain does not import DatabaseBackupModule

DatabaseBackupModule stays @Global, but its comment gave a vague reason.
The real one is concrete: the backup domain already reaches admin through
auth and settings, so importing the module from AdminModule would add a
domain cycle that lint:boundaries rejects. The module comment now says
that and points at the e2e harness that imports it by hand, and
AdminModule notes where its DATABASE_BACKUP comes from.

server/src/nest/admin/admin.module.ts

@@ -24,6 +24,10 @@ import { PackingModule } from '../packing/packing.module';
 // not @Global, so the import must be explicit.
 import { PermissionsModule } from '../permissions/permissions.module';
 import { AppConfigModule } from '../app-config/app-config.module';
+// AdminService and DemoResetJob inject DATABASE_BACKUP, which the @Global
+// DatabaseBackupModule (nest/backup/) provides once BackupModule is in the
+// graph. It is not imported here on purpose: see that module for the cycle an
+// import would close.
 import { Users } from '../../db/entities/Users.entity';
 import { AuditLog } from '../../db/entities/AuditLog.entity';
 import { AppSettings } from '../../db/entities/AppSettings.entity';

server/src/nest/backup/database-backup.module.ts

@@ -7,11 +7,15 @@ import { SqliteDatabaseBackup } from './sqlite-database-backup';
 /**
  * Binds the database backup port (`DATABASE_BACKUP`) to the engine in use.
  *
- * Global because its consumers sit in two domains that must not depend on each
- * other: the backup domain itself and the admin domain (the demo reset job and
- * the baseline route). They inject the token from the shared database kernel
- * and never import this file. `BackupModule` imports it, which is what puts it
- * in the application.
+ * Global because its consumers sit in two domains: the backup domain itself
+ * and the admin domain (the demo reset job and the baseline route). Admin
+ * cannot import this file. The backup domain already reaches admin (backup
+ * imports auth, auth imports settings, settings imports admin), so an
+ * `admin -> backup` import would close a new domain cycle that
+ * `lint:boundaries` refuses. Both inject the token from the shared database
+ * kernel instead. `BackupModule` imports this module, which is what puts it in
+ * the application; a test harness that mounts `AdminModule` without
+ * `BackupModule` imports it itself (tests/e2e/admin.e2e.test.ts).
  */
 @Global()
 @Module({