Add analysis preview to case list

Introduce a `preview_lines` setting in `users.toml` to control the
number of visible lines for the analysis preview in the case list. This
feature extracts the first paragraph of the `document.md` file, strips
markdown formatting, and displays it, capped by the configured
`preview_lines`. The actual line clamping is handled client-side via CSS
`line-clamp`.

server/tests/magic_link_test.rs

@@ -26,6 +26,7 @@ fn make_user() -> User {
         whisper: Default::default(),
         retention: Default::default(),
         window_hours: 72,
+        preview_lines: 2,
     }
 }