feat: support independent plugin project URLs

[OneBigMoon]

Summary

• expose optional project_url metadata for installed and marketplace plugins
• keep plugin_repo as a compatibility fallback for existing plugins
• preserve project_url when marketplace metadata is merged with installed plugin history

Verification

• targeted plugin metadata tests passed
• full suite: 2462 passed, 1 skipped, 14 subtests passed
• one unrelated existing failure remains in tests/test_metainfo.py: moviepilot-rust 0.2.8 returns UHD BluRay while the test expects UHD

PR-Agent 摘要

🤖 Generated by PR Agent at 27b96f1

• 新增插件独立项目主页字段
• 兼容旧版 plugin_repo 项目链接
• 合并市场元数据时保留主页
• 补充市场及本地插件测试

[github-actions]

PR-Agent Code Review

本次变更为已安装插件和市场插件新增可选的项目主页元数据，兼容本地插件原有的 plugin_repo 声明，并在市场元数据合并时保留该字段。未发现需要提出的具体行为缺陷。

审查提交：27b96f1