[Backend] Abstract Cloud File Storage Wrapper (S3)

[armorbreak001]

Fixes #295

What was done

• S3StorageProvider (backend/src/storage/s3-storage.provider.ts):
  • Uses @aws-sdk/client-s3 (AWS SDK v3) — modern, modular, tree-shakeable
  • Streaming uploads: Accepts Buffer or Readable stream, uses Node.js streams internally so 50MB+ files don't OOM your Pod
  • Upload method: Takes buffer/stream + original name + content type + optional folder prefix, returns { url, key, bucket }
  • Delete method: Remove objects by key
  • Presigned URLs: Generate temporary access URLs for private files
  • Content type auto-detection: Maps file extensions to MIME types (jpg/png/gif/webp/pdf)
  • Graceful degradation: If S3 credentials not in .env, isAvailable() returns false — no crashes
  • Configured via environment variables: AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, AWS_REGION, AWS_S3_BUCKET
• StorageController at POST /storage/upload:
  • Multer FileInterceptor with 50MB limit
  • MIME type filtering (images + PDF only)
  • Streams file buffer to S3 via S3StorageProvider
  • Returns public URL on success
• Added @aws-sdk/client-s3 and @aws-sdk/s3-request-presigner dependencies

How to verify

1. Add to backend .env:

   AWS_ACCESS_KEY_ID=your-key
   AWS_SECRET_ACCESS_KEY=your-secret
   AWS_REGION=us-east-1
   AWS_S3_BUCKET=stellar-guilds-uploads
   

2. cd backend && npm install
3. Start server: npm run start:dev
4. Test upload: curl -X POST http://localhost:3000/storage/upload -F "file=@/path/to/image.png"
5. Response should include a public S3 URL like: https://bucket.s3.region.amazonaws.com/uploads/uuid-filename.png
6. Without env vars configured, endpoint returns informative message (no crash)