feat: add step-by-step visual replay and harden process lifecycle

- UI: Implement dual-mode playback in FloatingVideoPlayer supporting video and step-by-step frame replay with scrub controls and auto-fallback
- Runtime: Prevent asyncio event loop hangs on Darwin and Linux by avoiding psutil.wait_procs on direct child processes
- Task Queue: Add worker exit watchdog and automatic recording recovery for completed or interrupted sessions
- Recording: Add ffmpeg ultrafast transcode fallback, shield recording finalization against cancellation, and handle on-the-fly MP4 conversion
- Docs: Polish README_CN styling and streamline flash runner agent prompt

apps/admin_console/database/repositories/session_repository.py

@@ -141,6 +141,22 @@ class SessionRepository:
         except Exception:
             return False
 
+    def mark_recording_ready(self, session_id: str, local_video_path: str) -> bool:
+        """Mark a recording as ready with its finalized video path."""
+        try:
+            with db_session(self.db_path) as conn:
+                cursor = conn.cursor()
+                cursor.execute(
+                    "UPDATE video_recordings SET status = 'ready', error = NULL, "
+                    "local_video_path = ?, end_time = COALESCE(end_time, ?) "
+                    "WHERE session_id = ?",
+                    (str(local_video_path), time.time(), str(session_id)),
+                )
+                conn.commit()
+                return cursor.rowcount > 0
+        except Exception:
+            return False
+
     def get_llm_traces_for_profile(self, session_id: str, limit: int = 3) -> list[str]:
         with db_session(self.db_path) as conn:
             cursor = conn.cursor()