Add S3 multipart upload support for the dev/minio backend

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Overview

Local dev runs on minio (S3), which has no GCS resumable session. Add S3 multipart support to the dev/minio backend so upload_url's resumable scheme works there, and the web frontend (#5988) can use one chunked-upload path everywhere.

Complexity: Medium
Target branch: unstable

Context

#5975 added the resumable scheme as a server-initiated GCS session; on the S3 backend upload_url still returns single-PUT. minio supports the S3 multipart API (initiate, upload parts, complete), an equivalent chunked, resumable-style upload.

Out of Scope

• Upload integrity enforcement on minio. This is dev-only, and mirrors today's S3 single-PUT, which does not enforce Content-MD5.

Acceptance Criteria

• On the S3 backend, the resumable upload_url path initiates an S3 multipart upload and returns what the client needs to upload parts and finalize.
• A file uploads to dev/minio in multiple parts via this path.
• The web frontend uses this path in dev with no single-PUT fallback.

References

• AWS S3 multipart upload overview: https://docs.aws.amazon.com/AmazonS3/latest/userguide/mpuoverview.html
• Presigned URLs for multipart upload with boto3: https://github.com/boto/boto3/issues/2305
• Browser multipart and presigned-URL reference implementation (S3/minio): https://github.com/prestonlimlianjie/aws-s3-multipart-presigned-upload

AI usage

Drafted with Claude (Opus 4.8), which researched the minio/S3 multipart references; I reviewed and edited.

Revised later with Claude (Opus 5) to put the #5975 prerequisite in past tense now that it has closed, and to remove a one-line "The Change" section that restated an acceptance criterion.

This issue centers on the S3 backend's resumable upload_url path and the web frontend integration; first read issue #5975 and the AWS multipart-upload references. Trace how upload_url currently handles S3 single-PUT uploads, then verify the acceptance criteria by uploading a file to dev/minio in multiple parts and confirming the frontend uses that path without fallback.