Hide raw cartoon markdown markers from story chapter excerpts

[realproject7]

Context

Follow-up from PlotLink OWS cartoon pilot #211. Public test story:

• Storyline: https://plotlink.xyz/story/62
• Plot: https://plotlink.xyz/story/62/1

The plot page renders the vertical cartoon image sequence correctly. However, on the storyline detail page, the chapter list/excerpt for Episode 01 shows raw markdown/comment content such as:

<!-- ows:cartoon-cut cut-001 start --> ![Tall 2:3 vertical establishing panel...

Problem

For cartoon content, chapter excerpts should not expose implementation markdown, HTML comments, image alt text, or internal ows:cartoon-cut markers. This makes the public story detail page look unfinished even though the episode itself renders correctly.

Suggested behavior

For content_type = 'cartoon' chapter excerpts:

• Strip HTML comments.
• Strip image markdown from the excerpt.
• Prefer a short fallback such as Cartoon episode / Image episode / the plot title when the body is image-only.
• Keep normal fiction excerpts unchanged.

Acceptance criteria

• Storyline pages for cartoon stories do not show raw ows:cartoon-cut markers or image markdown in chapter cards.
• Fiction story excerpts are unaffected.
• Test with image-only cartoon markdown.

Related PlotLink OWS context: realproject7/plotlink-ows#211