fix(replay): unify step indexing, dual-index entity deduplication, and prevent step-4 start bug

- Backend: maintain _step_number_cache in DataEngine and include step_number in all step_updated SSE events (summary, thinking, execution result)
- Admin console: query unassigned trace step_ids in stepless_rows to prevent orphan traces from being lost
- Showcase UI: implement dual-indexed entity deduplication (by step_id and step_number) in extractStepReplayFrames to eliminate phantom ghost steps
- Showcase UI: filter visual screenshot steps before generating sequential 1-based frame numbers, preventing step-4 start and index shift bugs
- Showcase UI: add rawStepNumber and summary properties to StepReplayFrame
- Showcase UI: update FloatingVideoPlayerComponent session reset effect to only reset playback on sessionId change rather than on every stepFrames mutation
- Showcase UI: add stepSeekRequest signal and stepIndex parameter to AgentService.openVideoPlayer
- Unit tests: add Karma and Pytest unit tests verifying step 1 start, dual-indexing, and SSE step_number payloads

apps/showcase_ui/src/app/components/floating-video-player/floating-video-player.component.ts

@@ -130,13 +130,20 @@ export class FloatingVideoPlayerComponent implements OnDestroy {
       { allowSignalWrites: true }
     );
 
-    // Reset step playback state when session changes
+    // Reset step playback state only when session actually changes
+    let lastTrackedSessionId: string | null = null;
     effect(
       () => {
-        const _ = this.stepFrames();
-        this.activeStepIndex.set(0);
-        this.viewMode.set('pre');
-        this.pauseStepPlay();
+        const curSessionId = this.agentService.currentSessionId();
+        const total = this.totalStepFrames();
+        if (curSessionId !== lastTrackedSessionId) {
+          lastTrackedSessionId = curSessionId;
+          this.activeStepIndex.set(0);
+          this.viewMode.set('pre');
+          this.pauseStepPlay();
+        } else if (total > 0 && this.activeStepIndex() >= total) {
+          this.activeStepIndex.set(total - 1);
+        }
       },
       { allowSignalWrites: true }
     );
@@ -171,6 +178,12 @@ export class FloatingVideoPlayerComponent implements OnDestroy {
         this.seek(target, true);
       }
     });
+
+    effect(() => {
+      const request = this.agentService.stepSeekRequest();
+      if (!request) return;
+      this.seekStepFrame(request.index);
+    });
   }
 
   public onLiveStreamError(): void {