feat: Cloud storage service (Cloudinary / Supabase)

[carvanino]

Summary

Integrate a cloud storage provider so uploaded files are stored and accessible via a public URL.

Tasks

• Create app/services/storage.py
• Integrate Cloudinary or Supabase Storage (pending team decision from tobbytobs)
• Accept file bytes + filename, upload to storage, return a public URL
• Handle upload failures with a descriptive error

Acceptance Criteria

• A valid file upload returns a public URL
• A storage failure raises a descriptive exception (does not swallow errors)
• Credentials are read from environment variables, never hardcoded

Blocker

Storage provider decision pending — ping tobbytobs for recommendation before starting