Remove the non-resumable single-PUT upload path

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Overview

Once the web (#5988) and ricecooker (learningequality/ricecooker#678) clients both upload via resumable, remove the single-PUT path so resumable is the only upload scheme.

Complexity: Low
Target branch: unstable

Context

#5975 added opt-in resumable via the resumable flag on upload_url, retaining single-PUT for un-migrated clients. Once both clients have migrated, single-PUT is dead code.

The Change

• Remove the presigned single-PUT URL subsystem in storage_common.py: get_presigned_upload_url, _get_gcs_presigned_put_url, _get_s3_presigned_put_url, and UnknownStorageBackendError if unused elsewhere.
• Remove the get_presigned_upload_url call from upload_url.
• Make resumable the only scheme, dropping the resumable flag and its branching along with the non-resumable >500 MB guard.
• Remove the presigned-URL tests in test_storage_common.py.
• Remove any remaining single-PUT client code.

Acceptance Criteria

• upload_url always returns a resumable session or definitive skip; no single-PUT URL is produced.
• The presigned single-PUT subsystem is gone: the storage_common.py helpers, the resumable flag, and the >500 MB guard.
• No client code performs single-PUT.

AI usage

Drafted with Claude (Opus 4.8) from the #5975 cutover plan; I reviewed and edited it.

Revised later with Claude (Opus 5) to resolve the bare client references into issue links.

Start in storage_common.py at upload_url and the named presigned-upload helpers, then review test_storage_common.py for the tests to remove. Search the repository for remaining single-PUT client code and the resumable flag. Done means upload_url only returns a resumable session or skips definitively, with no single-PUT helpers, guard, or client path remaining.