feat: remote asset pipeline

[d12frosted]

Summary

Upload attachments to external storage, rewrite URLs in published content.

Upload Targets

• S3-compatible storage
• Custom upload function (user-defined)

URL Rewriting

• Replace local attachment paths with remote URLs
• Handle relative vs absolute paths
• Cache uploaded asset URLs to avoid re-upload

Asset Tracking

• Know which assets are used by which outputs
• Clean up orphaned remote assets (optional, dangerous)

Configuration

(porg-project
 ...
 :assets
 (:upload-fn #'my-s3-upload
  :url-prefix "https://assets.barberry.io/"
  :cache-file "~/.cache/porg-assets.el"))

Open Questions

• Sync vs async upload?
• How to handle upload failures mid-build?
• Image optimisation as part of pipeline? (Resize, compress)