feat(sharing): allow custom share IDs

[MuelNova]

Description / 描述

Allow users to set custom share IDs when creating or updating shares, instead of
being limited to random 8-12 character IDs.

Changes:

• Expand sharing ID column from char(12) to varchar(64) to support longer custom
  IDs
• Add new_id field to UpdateSharingReq for renaming existing share IDs
• Add server-side ID validation: max 64 characters, allowing letters, numbers,
  underscores, hyphens, and CJK characters
• Add duplicate ID conflict check when updating share IDs
• Add customize_share_id permission (bit 15) to control which users can set custom
  IDs
• Cache invalidation on ID update

允许用户在创建或更新分享时设置自定义分享 ID，而不是只能使用随机生成的 8-12 位 ID。

Motivation and Context / 背景

Random share IDs like aB3xK9mQ are hard to remember. Custom IDs (e.g. my-photos,
project-docs) make share links more readable and memorable.

随机分享 ID 难以记忆，自定义 ID 让分享链接更易读、更好记。

Closes #1806

How Has This Been Tested? / 测试

• Verified GORM AutoMigrate correctly alters column from char(12) to
  varchar(64)
• Tested custom ID creation via API with valid/invalid IDs
• Tested ID rename via new_id field in update request
• Tested duplicate ID rejection
• Tested permission check: users without customize_share_id permission cannot set
  custom IDs

Checklist / 检查清单

• I have read the
  CONTRIBUTING
  document.
• I have formatted my code with go fmt or prettier.
• I have added appropriate labels to this PR (or mentioned needed labels in the
  description if lacking permissions).
  Labels needed: enhancement, sharing
• I have requested review from relevant code authors using the "Request review"
  feature when applicable.
• I have updated the repository accordingly (If it's needed).
  • feat(sharing): add custom share ID input and permission OpenList-Frontend#472
  • OpenList-Docs #XXXX

[jyxjjj]

⚠️ Reverted as a mistake

Changing a DB column requires a schema migration/upgrade, which is not handled in this PR.

Please include proper migration codes.

[jyxjjj]

I'm sorry for my mistake, I overlooked the AutoMigrate.

[MuelNova]

I'm sorry for my mistake, I overlooked the AutoMigrate.

Yes, GORM will handle this automatically. Thank you for your reviewing :D