feat(background-task): rendu live des tâches de fond — subscriber UI + canal IPC (#58)

Câble un canal attachable au flux de sortie d'une tâche de fond (runner
infrastructure + commande app-tauri + port/adaptateurs frontend) et le
panneau ProjectWorkStatePanel s'y abonne pour un rendu live au lieu d'un
état figé au dernier snapshot.

Validations obtenues avant commit :
- cargo test -p infrastructure --test background_task_runner : vert
- cargo check -p backend -p app-tauri : vert
- npx vitest run src/features/workstate/workstate.test.tsx : vert
- npm run typecheck : vert
- verdict QA #58 : vert

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

crates/backend/src/dto.rs

@@ -562,6 +562,20 @@ pub struct ReattachResultDto {
     pub scrollback: Vec<u8>,
 }
 
+/// Response DTO for `attach_background_task`: the retained bytes to repaint for
+/// a background command task before optional live bytes arrive on the channel.
+#[derive(Debug, Clone, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct AttachBackgroundTaskResultDto {
+    /// The task that was attached (echoed back for the frontend).
+    pub task_id: String,
+    /// Recent output bytes: PTY scrollback for a live task, persisted tail for a
+    /// terminal task.
+    pub scrollback: Vec<u8>,
+    /// Whether a live PTY subscription was installed for subsequent output.
+    pub live: bool,
+}
+
 /// Request DTO for `write_terminal`.
 #[derive(Debug, Clone, Deserialize)]
 #[serde(rename_all = "camelCase")]