New uploads route to R2 with folder selection (uploads/images/guides/finished).
Existing local files continue to be served via express.static for backward compat.
Adds R2_ACCOUNT_ID, R2_ACCESS_KEY_ID, R2_SECRET_ACCESS_KEY, R2_BUCKET,
R2_PUBLIC_URL env vars. listUploadedAssets merges R2 objects + local files.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
