feat: Add files module — VueFinder-compatible REST API over NATS

Implements GET/POST /api/files proxy for all VueFinder operations
(list, search, preview, download, delete, rename, move, copy, mkdir,
new-file, save, upload). Routes via NATS request-reply to the companion
agent on corrosion.{license_id}.files.cmd with 30s timeout. Gated
behind files.view (GET) and files.manage (POST) permissions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend-nest/src/app.module.ts

@@ -34,6 +34,7 @@ import { AdminModule } from './modules/admin/admin.module';
 import { SetupModule } from './modules/setup/setup.module';
 import { MigrationModule } from './modules/migration/migration.module';
 import { ChangelogModule } from './modules/changelog/changelog.module';
+import { FilesModule } from './modules/files/files.module';
 
 // Shared Services
 import { NatsService } from './services/nats.service';
@@ -103,6 +104,7 @@ import { NatsBridgeGateway } from './gateways/nats-bridge.gateway';
     SetupModule,
     MigrationModule,
     ChangelogModule,
+    FilesModule,
   ],
   providers: [
     // Global guards (order matters: auth first, then license, then permissions)