chore(deps): migrate from deprecated s3/manager to feature/s3/transfermanager

[jhagberg]

aws/aws-sdk-go-v2/service/s3/manager is deprecated in favour of
aws/aws-sdk-go-v2/feature/s3/transfermanager
(aws/aws-sdk-go-v2#3306). golangci-lint
has been flagging this as five SA1019 hits on main for several CI
runs, so every PR inherits a failing Lint code check.

Where it hits

• upload/upload.go:133 — manager.NewUploader(s3Client)
• upload/upload.go:224 — uploader.Upload(ctx, …)
• upload/upload.go:229 — func(u *manager.Uploader) option callback
• list/list_test.go:90 — manager.NewUploader(s3Client) (test fixture)
• list/list_test.go:94 — uploader.Upload(context.Background(), …) (test fixture)

Scope

• Swap imports to feature/s3/transfermanager.
• Port uploader construction (constructor signature differs — the new
  package is instance-style with transfermanager.New(s3Client, opts...)).
• Port the Uploader options callback (progress-bar wiring in
  upload.go:229 — confirm the equivalent hook on the new type).
• Update the test fixture in list/list_test.go to match.
• Re-run golangci-lint run --timeout 5m locally to confirm main
  goes green before merge.

Out of scope

• Behavioural changes to upload chunking, concurrency, or retry
  semantics beyond what the new package's defaults enforce. If the
  migration requires tuning to match current behaviour, keep that
  within this PR; larger rewrites belong in a follow-up.

Why now

Having a failing Lint code check on main makes it hard to use
lint as a merge gate on downstream work (see the #663 stack, starting
with #679). Unblocking this first keeps every stacked PR's CI signal
honest.

[jhagberg]

Duplicate of #658 which already tracks the same 5 SA1019 hits in upload/upload.go and list/list_test.go. Closing in favour of the existing issue.