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/transcribe_test.rs

@@ -267,6 +267,7 @@ fn test_config_with_whisper(whisper_url: String) -> Arc<Config> {
             whisper: Default::default(),
             retention: Default::default(),
             window_hours: 72,
+            preview_lines: 2,
         }],
         whisper_url,
         whisper_timeout_seconds: 5,