Add Title Dedup third-party extension

[chenhaoc]

Summary

• add the Title Dedup repository to repositories.json
• add the generated extension metadata to extensions.json
• list the extension in the third-party section of README.md

Extension

Repository: https://github.com/chenhaoc/freshrss-dedup-extension

This extension marks target feed entries as read when the same title already exists in source feeds.

[Alkarex]

Thanks. Another PR has created merge conflicts. Could you please fix them?

[chenhaoc]

Conflicts resolved by merging the latest main branch. The final diff still only updates README.md, extensions.json, and repositories.json for the Title Dedup listing.