fix: avoid cover thumbnail DB deadlock

[navels]

Summary

• defer cover thumbnail regeneration until after the Calibre metadata edit transaction commits
• pass the committed book path and last-modified timestamp into single-book thumbnail refreshes
• avoid reopening metadata.db for that single-book refresh path when committed snapshot data is available
• add regression coverage for the ordering/snapshot contract

Testing

• pytest tests/unit/test_thumbnail_deadlock_regression.py -q
• python -m py_compile cps/editbooks.py cps/helper.py cps/tasks/thumbnail.py tests/unit/test_thumbnail_deadlock_regression.py

Refs #1256 and #1256 (comment)

[new-usemame]

Backported into Calibre-Web-NextGen v4.0.46 at 34f1985. Drop-in image for users hitting this: ghcr.io/new-usemame/calibre-web-nextgen:latest. Thanks @navels.