fix(upload): respect overwrite for direct uploads

[jyxjjj]

Summary / 摘要

• 修复 HTTP Direct Upload 获取直传信息时未向底层传递 Overwrite 语义的问题。

• 覆盖上传时允许已存在目标继续生成直传信息，与 stream 上传覆盖行为保持一致。

• 非覆盖上传时按完整目标路径检查同名文件，并继续返回 file exists。

• This PR has breaking changes.
  / 此 PR 包含破坏性变更。

• This PR changes public API, config, storage format, or migration behavior.
  / 此 PR 修改了公开 API、配置、存储格式或迁移行为。

• This PR requires corresponding changes in related repositories.
  / 此 PR 需要关联仓库同步修改。

Related repository PRs / 关联仓库 PR:

• OpenList-Frontend:
• OpenList-Docs:

Related Issues / 关联 Issue

Fixes #2102

Testing / 测试

• go test ./...
• go test ./internal/fs ./internal/op ./server/handles ./server/s3 ./drivers/s3
• Manual test / 手动测试:

Checklist / 检查清单

• I have read CONTRIBUTING.
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• I have formatted the changed code with gofmt, go fmt, or prettier where applicable.
  / 我已按适用情况使用 gofmt、go fmt 或 prettier 格式化变更代码。
• I have requested review from relevant maintainers or code owners where applicable.
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[Copilot]

Pull request overview

This PR fixes direct-upload overwrite semantics by propagating the Overwrite intent through the HTTP Direct Upload API down to the storage-layer direct uploader, aligning behavior with stream uploads and addressing issue #2102 (S3/R3 “object already exists” blocking direct upload info).

Changes:

• Add an overwrite parameter to GetDirectUploadInfo across server -> internal/fs -> internal/op layers.
• Skip the “object already exists” pre-check when overwrite is enabled, allowing direct upload info generation for existing targets.
• Fix the non-overwrite existence check in the direct upload handler to check the full destination path (dir + file name).

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
server/s3/redirect.go	Passes overwrite intent when generating S3 direct-upload redirect URLs.
server/handles/direct_upload.go	Uses overwrite header to decide existence checks and passes overwrite through to FS direct upload info.
internal/op/fs.go	Adds overwrite-aware existence probing before generating driver direct upload info.
internal/fs/put.go	Threads overwrite through the internal FS direct-upload plumbing.
internal/fs/fs.go	Updates exported internal FS API to accept overwrite and forwards it.

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[Copilot]

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.