Fix: --upload-blobs without storage connections (#5)

[jensens]

Summary

• Add standalone S3 CLI flags (--s3-bucket, --s3-endpoint-url, --s3-region, --s3-access-key, --s3-secret-key)
• --upload-blobs creates an S3Client directly from flags — no source or destination storage connections opened
• Can now run in parallel with an ongoing --deferred-blobs import (no PG contention)
• Falls back to opening destination storage if S3 flags not provided

Fixes #5

Usage

zodb-convert --upload-blobs /tmp/manifest.tsv \
  --s3-bucket my-bucket \
  --s3-endpoint-url https://fsn1.your-objectstorage.com \
  --s3-access-key $KEY \
  --s3-secret-key $SECRET \
  -w 16

Test plan

• 87 tests pass, 94.3% coverage

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