refactor: harden task management and TypeScript safety

[overtrue]

Pull Request

Description

Hardens the TypeScript codebase with a focused TaskManager redesign. Retry scheduling now respects concurrency and limits, task cleanup no longer cancels active work, handler types preserve task-kind relationships, and manager lifecycle cleanup is explicit.

Also fixes related upload/delete error handling, credential-change races, request logging safety, object preview/download guards, and several smaller type-safety issues found during the audit.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Performance improvement
• Test improvements
• Security fix

Testing

• Unit tests added/updated
• Manual testing completed

pnpm install --frozen-lockfile
pnpm type-check
pnpm lint
pnpm prettier --check .
pnpm test:run
pnpm build

Checklist

• Code follows the project's style guidelines
• Self-review completed
• TypeScript types are properly defined
• All commit messages are in English (Conventional Commits)
• All existing tests pass
• No new dependencies added, or they are justified

Related Issues

N/A

Screenshots (if applicable)

N/A

Additional Notes

The full test suite passes with 300 tests.

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