fix(share): remove share when user delete

[ILoveScratch2]

Description / 描述

在删除用户时同步删除分享

Motivation and Context / 背景

在用户删除但分享存在的情况下，获取用户访问无效

Closes #1491

How Has This Been Tested? / 测试

Checklist / 检查清单

• I have read the CONTRIBUTING document.
  我已阅读 CONTRIBUTING 文档。
• I have formatted my code with go fmt or prettier.
  我已使用 go fmt 或 prettier 格式化提交的代码。
• I have added appropriate labels to this PR (or mentioned needed labels in the description if lacking permissions).
  我已为此 PR 添加了适当的标签（如无权限或需要的标签不存在，请在描述中说明，管理员将后续处理）。
• I have requested review from relevant code authors using the "Request review" feature when applicable.
  我已在适当情况下使用"Request review"功能请求相关代码作者进行审查。
• I have updated the repository accordingly (If it’s needed).
  我已相应更新了相关仓库（若适用）。
  • OpenList-Frontend #XXXX
  • OpenList-Docs #XXXX

[Copilot]

Pull Request Overview

This PR fixes a data consistency issue by ensuring that when a user is deleted, all their associated shares are also deleted to prevent accessing invalid user data.

• Adds cascading deletion of shares when a user is deleted
• Adds null-safe check in sharing validation
• Implements new database and operation layer functions for bulk share deletion

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
internal/op/user.go	Adds call to delete user's shares before user deletion and imports errors package
internal/op/sharing.go	Adds new function to delete shares by creator ID
internal/model/sharing.go	Adds null check for Creator field in validation
internal/db/sharing.go	Implements database function to delete shares by creator ID

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