Integrate cloud storage targets (S3, etc.)

[jeisaacs]

Summary

Add optional connections to cloud storage providers so source PDFs and generated outputs can live outside the local machine.

Why

• Supports teams collaborating from different devices
• Reduces manual download/upload steps for enterprise workflows
• Enables automated cleanup policies driven by the storage backend

Proposal

• Start with S3-compatible storage for uploads and output persistence
• Provide configuration via environment variables and UI toggles (local vs. cloud)
• Ensure sensitive credentials are never exposed to the frontend

Acceptance Criteria

• Users can opt into saving outputs to an S3 bucket (or compatible service)
• Backend streams files directly to storage and provides signed download links
• Local-only mode continues to function by default
• Documentation covers setup, permissions, and fallback behavior