feat: 优化 GitHub Actions 合并 PDF 时的文件名生成规则

[hect0x7]

原来使用简单的 Aid 或 Pid 进行命名，用户反馈导出的 PDF 名字全是数字看不出是什么。现在：

1. 默认情况下，本子维度的 PDF 命名为 Atitle，章节维度的 PDF 命名为 Atitle_Ptitle。

2. 新增可选的环境变量和 Action 面板输入参数 PDF_NAME_RULE，允许用户自定义生成规则。

Summary by CodeRabbit

• New Features
  • Added configurable PDF filename naming rule option to workflow dispatch with sensible defaults for different merge strategies.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: c51e470b-1779-40de-b013-db0e6358b476

📥 Commits

Reviewing files that changed from the base of the PR and between 145af2a and 62e308c.

📒 Files selected for processing (2)

• .github/workflows/download_dispatch.yml
• usage/workflow_download.py

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📝 Walkthrough

Walkthrough

This PR introduces a configurable PDF naming rule for merged PDFs via a new GitHub Actions workflow dispatch input PDF_NAME_RULE. The input is propagated to the job environment and used by the PDF plugin configuration, with sensible defaults based on the merge dimension (album vs. photo).

Changes

Cohort / File(s)	Summary
GitHub Actions Workflow Configuration .github/workflows/download_dispatch.yml	Added new workflow dispatch input PDF_NAME_RULE (string type, optional, default empty string) and propagated it to job environment for downstream consumption.
PDF Filename Rule Configuration usage/workflow_download.py	Updated cover_option_config to use configurable filename_rule from PDF_NAME_RULE environment variable, with defaults of [{Aid}] {Atitle} for album-dimension and [{Aid}] {Pindex}_{Pid}_{Ptitle} for chapter-dimension merging.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Possibly related PRs

• Dev #489: Modifies the same PDF plugin filename rule logic in usage/workflow_download.py, providing context for how the PDF_NAME_RULE configuration is integrated with the Img2pdf plugin.

Poem

🐰 A naming rule hops into the fold,
Where PDFs now dance to patterns untold,
Album or chapter, the choice is quite clear,
Customizable names make workflows more dear! 📄✨

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-pdf-sort

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[github-actions]

To prevent beginners from mistakenly submitting PRs,
if your PR only modifies the usage/workflow_download.py file,
it will be automatically closed.
If you really want to submit a PR, please reopen it yourself.
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