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/modules/files/files.module.ts

@@ -0,0 +1,10 @@
+import { Module } from '@nestjs/common';
+import { FilesController } from './files.controller';
+import { FilesService } from './files.service';
+import { NatsService } from '../../services/nats.service';
+
+@Module({
+  controllers: [FilesController],
+  providers: [FilesService, NatsService],
+})
+export class FilesModule {}