Parallelize initiate and complete directory multipart upload

[GianlucaFicarelli]

Parallelize multipart upload directory initiate and complete.

Implement #669 with threads.

---

Speeds up directory uploads by running S3 CreateMultipartUpload and
CompleteMultipartUpload calls concurrently across files, instead of
sequentially.

Changes

app/service/asset_helpers.py (new file)

• Extracted pure, thread-safe S3 helpers out of asset.py:
  initiate_multipart_upload, complete_asset_s3, complete_empty_asset_s3,
  generate_upload_presigned_urls, build_asset_read_with_upload_meta

app/service/asset.py

• Directory multipart initiate: DB record creation remains sequential; S3
  initiation is now parallelized via ThreadPoolExecutor
• Directory multipart complete: S3 completion/verification is now parallelized
  via ThreadPoolExecutor
• Removed the now-redundant entity_asset_empty_upload_initiate and
  entity_asset_empty_upload_complete functions (logic absorbed into helpers)
• create_entity_asset_unverified now returns Asset (ORM object) instead of
  AssetRead, avoiding redundant serialization in callers

app/utils/s3.py

• get_s3_client now returns a thread-local client (one per storage type per
  thread) using boto3.session.Session(), avoiding concurrent access to the
  shared default session

app/utils/virtual_lab.py

• Extracted resolve_virtual_lab_id helper to deduplicate repeated
  virtual-lab-id resolution logic across asset service functions

app/config.py

• Added S3_DIRECTORY_UPLOAD_MAX_WORKERS setting (default: 32)

[codecov]

Codecov Report

❌ Patch coverage is 98.26087% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
app/service/asset.py	94.87%	0 Missing and 2 partials ⚠️

Flag	Coverage Δ
pytest	97.75% <98.26%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
app/config.py	97.89% <100.00%> (+0.02%)	⬆️
app/service/admin.py	90.38% <100.00%> (ø)
app/service/asset_helpers.py	100.00% <100.00%> (ø)
app/utils/s3.py	88.70% <100.00%> (+0.37%)	⬆️
app/utils/virtual_lab.py	100.00% <100.00%> (ø)
app/service/asset.py	98.93% <94.87%> (-1.07%)	⬇️

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[mgeplf]

LGTM