add local ciphertext storage module to backend.

Bind-mount backend/uploads in dev; path helpers with traversal guards and Jest coverage for StorageService.

backend/src/AppModule.ts

@@ -3,6 +3,7 @@ import { ConfigModule, ConfigService, registerAs } from '@nestjs/config';
 import { TypeOrmModule } from '@nestjs/typeorm';
 import { getAppConfig, getPostgresConfig } from './config';
 import { HealthCheckModule } from './modules/healthCheck/HealthCheckModule';
+import { StorageModule } from './modules/storage/StorageModule';
 import { UploadModule } from './modules/upload/UploadModule';
 import { Config } from './types/Config';
 
@@ -17,6 +18,7 @@ import { Config } from './types/Config';
             inject: [ConfigService]
         }),
         HealthCheckModule,
+        StorageModule,
         UploadModule
     ]
 })