[BE-06] Set up AWS S3 file upload service with Multer for asset documents and images

[yusuftomilola]

Overview

Asset documents (invoices, warranties, manuals) and images need to be stored in cloud object storage. @aws-sdk/client-s3 and multer are installed but no upload service exists. This is a dependency for asset document management (BE-25) and user avatar uploads (BE-16).

Context

• package.json has @aws-sdk/client-s3@3.975.0 and multer@2.0.2
• backend/.env.example does not yet have S3 env vars — they need to be added
• Uploaded files must be stored under unique UUID-based keys, never original filenames (prevents path traversal)
• Files must not be publicly accessible by default — use pre-signed URLs for retrieval

Acceptance Criteria

• Add AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, AWS_REGION, AWS_S3_BUCKET to backend/.env.example
• Create src/file/file.service.ts with upload(file: Express.Multer.File, folder: string): Promise<string> returning the S3 object key
• Create file.service.ts method getPresignedUrl(key: string, expiresInSeconds: number): Promise<string>
• Create file.service.ts method delete(key: string): Promise<void>
• Create a reusable Multer config that whitelists MIME types: image/jpeg, image/png, image/webp, application/pdf and enforces a 10MB max file size
• Register FileModule in AppModule and export FileService for use by other modules

[portableDD]

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