feat(cache): add configurable HybridCache policies

[vxtls]

Summary / 摘要

• Add a user-selectable cache_policy configuration with auto, memory, and disk modes. The default remains auto, and the policy can also be set through OPENLIST_CACHE_POLICY.

• Improve auto mode so known-size workloads are admitted to memory only when the complete workload ceiling passes the memory check; otherwise, the cache uses disk from the start. Unknown-size workloads use disk in auto mode.

• Preserve HybridCache's existing memory-to-disk spill mechanism for workloads admitted by auto, while making memory and disk strict policies that do not silently switch backing stores.

• Add per-instance cache policy overrides for file streams and downloaders so callers can reuse HybridCache with workload-specific policy decisions without changing global configuration.

• Route unknown-size streams through HybridCache's sequential writer and bound downloader admission by its maximum concurrent working set instead of the complete download size.

• Enforce logical memory ceilings independently of page-aligned allocations and add coverage for policy parsing, backing-store selection, spill behavior, stream overrides, cleanup, and downloader ceilings.

• 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: N/A
• OpenList-Docs: N/A [WIP]

Related Issues / 关联 Issue

Closes #3073

Testing / 测试

• go test ./...
  • The full suite was attempted but did not pass because the unrelated existing TestNewOSSClientUsesEnvironmentHTTPSProxy test expected *http.Transport and received *net.safeTransport.
• go test ./internal/cache ./internal/conf ./internal/hybrid_cache ./internal/stream
• go test ./internal/net -run '^TestDownloaderMemoryCeiling$'
• go vet ./internal/cache ./internal/conf ./internal/hybrid_cache ./internal/net ./internal/stream
• go build ./...
• Manual test / 手动测试: Not performed.

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, 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.
  / 我已在适用情况下请求相关维护者或代码所有者审查。

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:

• ChatGPT
• Codex
• GitHub Copilot
• Claude
• Gemini
• Other (please specify) / 其他（请注明）:

Usage scope / 使用范围:

• Code generation / 代码生成

• Refactoring / 重构

• Documentation / 文档

• Tests / 测试

• Translation / 翻译

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• I have reviewed and validated all AI-assisted content included in this PR.
  / 我已审核并验证此 PR 中的所有 AI 辅助内容。

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• I can reproduce all AI-assisted content included in this PR without any AI tools.
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