feat(local): add PDF thumbnails on macOS

[flyingrtx2333]

Summary / 摘要

• Add an opt-in pdf_thumbnail setting to the Local driver. It is disabled by default.
• On macOS, render the first page of PDFs with the system Quick Look tool (/usr/bin/qlmanage).
• Reuse the Local driver's existing signed thumbnail URLs, concurrency limiter, image resize path, and disk cache.
• Keep PDF thumbnails unadvertised on unsupported platforms; existing image and video thumbnail behavior is unchanged.
• Propagate request cancellation to Quick Look and enforce a 30-second render timeout.

Operational notes:

• The existing thumbnail master switch must also be enabled.

• Rendering invokes the macOS Quick Look process for each cache miss; configuring thumb_cache_folder is recommended.

• Because PDF parsing consumes local resources, administrators should enable this selectively for untrusted or public storage.

• The setting is visible on all platforms but only advertises PDF thumbnail URLs on Darwin builds.

• 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: Not required; driver addition fields are rendered from backend metadata.
• OpenList-Docs: Not required for the initial implementation.

Testing / 测试

• go test ./...
  • The Local driver passes. The full command still fails on the current clean main baseline in unrelated packages, including Go 1.27 format-string vet failures in drivers/123, drivers/189, and drivers/189pc, plus an existing nil-pointer failure in drivers/onedrive_sharelink.
• Manual test / 手动测试:
  • GOTOOLCHAIN=local go test -race ./drivers/local — 8 tests passed.
  • GOTOOLCHAIN=local go vet ./drivers/local — passed.
  • CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go test -c ./drivers/local — produced a Linux ELF test binary.
  • CGO_ENABLED=0 GOOS=windows GOARCH=amd64 go test -c ./drivers/local — produced a Windows PE test binary.
  • bash build.sh dev web followed by a native Darwin arm64 build — passed with the rolling frontend embedded.
  • Isolated end-to-end smoke test with a copied database and synthetic PDF: /api/fs/list returned a signed thumb; fetching it returned HTTP 200, Content-Type: image/png, valid PNG magic bytes, and one disk-cache entry.
  • The Darwin integration test creates a PDF with a Unicode/space-containing filename via cupsfilter, renders it with Quick Look, and decodes the output dimensions.

Checklist / 检查清单

• I have read CONTRIBUTING.
  / 我已阅读 CONTRIBUTING。
• I confirm this contribution follows the repository license, contribution policy, and code of conduct.
  / 我确认此贡献符合仓库许可证、贡献规范和行为准则。
• I have formatted the changed code with gofmt.
  / 我已使用 gofmt 格式化变更。
• I have requested review from relevant maintainers or code owners where applicable.
  / Draft 阶段暂未指定维护者审查。

AI Disclosure / AI 使用声明

• This PR includes AI-assisted content.
  / 此 PR 包含 AI 辅助内容。

Tools used / 使用工具:

• Codex

Usage scope / 使用范围:

• Code generation / 代码生成

• Refactoring / 重构

• Documentation / 文档

• Tests / 测试

• Review assistance / 审查辅助

• I have reviewed and validated all AI-assisted content included in this PR.
  / The human collaborator will review the draft before it is marked ready.

• I have ensured that all AI-assisted commits include Co-Authored-By attribution.
  / AI 辅助提交包含 Co-Authored-By 归属信息。

• I can reproduce all AI-assisted content included in this PR without any AI tools.
  / This remains for the human collaborator to confirm before marking the PR ready.

The feature request and implementation were prompted and validated by the human collaborator on a real native macOS OpenList deployment. The submitted code, tests, PR text, and pre-submission review were prepared with Codex assistance. No private deployment paths, credentials, media filenames, or screenshots are included.