Expand direct image links instead of showing nothing (#43 follow-up)

A URL that points straight at an image file (Content-Type: image/*) has
no HTML to scrape Open Graph tags from, so the fetch found nothing and
the message showed no preview at all -- reported against
https://imgs.xkcd.com/comics/creepy.png.

link_preview_service now recognizes an allowed image content-type (same
list storage.py uses for uploads) before falling through to the HTML/og:
path, and returns the URL itself as the preview (LinkPreview.is_image).
No need to download the body -- the already-SSRF-validated URL is the
image. The frontend renders that case as a real expandable image
(message-image + lightbox, same as an actual attachment) instead of the
small title+description card, which would have nothing to show anyway.

Verified end-to-end against the reported URL.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/schemas/message.py

@@ -27,6 +27,7 @@ class LinkPreviewInfo(BaseModel):
     description: str | None
     image_url: str | None
     site_name: str | None
+    is_image: bool
 
 
 class MessageRead(BaseModel):