Refactor image upload structure

[slytter]

Current Problem

Uploads are broken in production due to Vercel serverless limitations:

• 413 FUNCTION_PAYLOAD_TOO_LARGE: Vercel enforces 4.5MB request body limit at infrastructure level
• 503 SERVICE_UNAVAILABLE: Remix's unstable_parseMultipartFormData crashes with multiple files on Vercel
• Memory issues: Buffering approach (request.formData()) hits memory limits with large files

Current workaround: Upload 1 file at a time with batch size = 1 (slow but works)

Proposed Solution

Implement direct client-to-BunnyCDN uploads with a two-phase storage strategy:

Phase 1: Temporary Upload (Draft)

• Client uploads directly to BunnyCDN (bypasses Vercel limits entirely)
• Files stored in a temp/ folder with 24h TTL
• Use short-lived signed URLs or similar for security
• Returns CDN URL immediately for preview

Phase 2: Persist on Save (Authenticated)

• When user saves project (authenticated), server moves files from temp/ to permanent storage
• Only persisted files count against storage quota
• Unused temp files auto-delete after 24h

Benefits

• No file size limits
• Faster uploads (direct to CDN)
• No Vercel memory/timeout issues
• Clean up abandoned uploads automatically
• Better UX (instant preview)

Tasks

• Research BunnyCDN options for temporary storage / signed URLs
• Implement client-side direct upload
• Create server endpoint for persisting files on project save
• Set up cleanup for expired temp files
• Update create.upload.tsx to use new flow