YouTube links do not generate link previews in v0.31.0

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Describe the bug

YouTube links do not generate a link preview.

For example: https://www.youtube.com/watch?v=iFmphiZhIKA

After adding this URL to a memo, only the link itself is displayed and no preview card is generated. Other links can generate previews as expected. I can reproduce the same behavior both on my self-hosted Memos instance and on the official Memos demo. Expected behavior: YouTube links should generate a preview card with the video title and thumbnail.

Steps to reproduce

1. Create a new memo.
2. Add this URL: https://www.youtube.com/watch?v=iFmphiZhIKA
3. Save the memo.
4. Observe that no link preview is generated.

Memos version

v0.31.0

Additional context

In v0.30.0, Memos had a YouTube-specific fallback for www.youtube.com/watch URLs that generated the thumbnail from img.youtube.com. The link preview implementation was refactored in v0.31.0, and that YouTube-specific fallback no longer appears to be present.

It may also be worth checking common YouTube URL variants such as:

• https://www.youtube.com/watch?v=VIDEO_ID
• https://youtu.be/VIDEO_ID
• https://m.youtube.com/watch?v=VIDEO_ID

so link previews work consistently regardless of which standard YouTube URL format is used.