Summary

Add presigned URL upload flow so large files (video, audio, documents) bypass
Vercel's serverless function body size limit (4.5MB) by uploading directly to
Supabase Storage from the browser.

Changes

• Add POST /ycode/api/files/presign route to generate signed upload URLs
• Add POST /ycode/api/files/register route to create Asset records after direct upload
• Update uploadFileApi to use presigned URLs for files >4.5MB, keeping server-side WebP conversion for small images
• Extract shared upload helpers (MAX_UPLOAD_FILE_SIZE, generateStoragePath, getDisplayName, validateCategoryMimeType) to reduce duplication across upload routes
• Refactor upload/route.ts to use shared validation and constants

Test plan

• Upload a small image (<4.5MB) — should go through server route and get WebP conversion
• Uploaa large video (>4.5MB, <50MB) — should use presigned URL flow without hitting FUNCTION_PAYLOAD_TOO_LARGE
• Upload a file >50MB — should be rejected with size error
• Verify uploaded assets appear in the File Manager with correct metadata
• Test upload with category filter (e.g., videos-only) — wrong types should be rejected