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/upload_test.rs

@@ -24,6 +24,7 @@ fn test_config() -> Arc<Config> {
             whisper: Default::default(),
             retention: Default::default(),
             window_hours: 72,
+            preview_lines: 2,
         }],
         api_keys: HashMap::from([("test-key-123".into(), "dr_test".into())]),
         ..Config::test_default()