Upload files to S3

Upload one or more files to the S3 bucket associated with the given website (STAGING).

• Images (PNG, JPG, JPEG, GIF, BMP, TIFF, WEBP) are limited to 1MB and auto-converted to WebP
• Videos (MP4, WEBM, MOV, AVI) are limited to 30MB (nginx body cap)
• PDFs are limited to 10MB. Every other type is refused.

Returns {"uploaded": [url, ...], "failed": [{"filename", "erro"}, ...]}. Validation (type/size/count) is all-or-nothing and happens BEFORE any write, so a rejected file leaves nothing in S3; failed only carries per-file S3 errors that occur mid-write.