sprint: fixes batch — saveNote history, DeepSeek retry, channel rotation, test fixes, docker env, blocking transcript, sync errors

backend/summarizer.py

@@ -62,9 +62,9 @@ def download_transcript(video_id: str) -> str:
         log.warning(f"Failed to fetch transcript for {video_id}: {e}")
         return None
 
-async def summarize_video(video_id: str) -> str:
-    # 1. Try transcript, fall back to description
-    transcript = download_transcript(video_id)
+async def summarize_video(video_id: str) -> str | None:
+    # 1. Try transcript, fall back to description (offload sync call to thread)
+    transcript = await asyncio.to_thread(download_transcript, video_id)
     
     if not transcript:
         # Fallback: use video description from YouTube API
@@ -120,7 +120,7 @@ async def summarize_video(video_id: str) -> str:
             summary = data["choices"][0]["message"]["content"]
         except Exception as e:
             log.error(f"DeepSeek API request failed for {video_id}: {e}")
-            summary = f"Error generating summary: {e}"
+            return None  # Leave summary NULL for retry on next sync
 
     # 3. Save transcript & summary to db
     async with aiosqlite.connect(DB_PATH) as db: