feat: Delete vfolder in bgtask

[fregataa]

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations

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📚 Documentation preview 📚: https://sorna--2739.org.readthedocs.build/en/2739/

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📚 Documentation preview 📚: https://sorna-ko--2739.org.readthedocs.build/ko/2739/

[fregataa]

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• feat: Delete vfolder in bgtask #2739 👈
• feat: Add log-level to background task message #2738 : 1 other dependent PR (#2737 )
• main

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[fregataa]

In #3060, we decided to convert folder deletion from blocking response to event-driven pattern rather than applying bgtask.