feat: notes + agent queue, frontend rebuild, pip mirror, gitignore sync_rotation.json

- Add video_notes table and agent_queue table for persistent notes and AI agent tasks
- Add API endpoints: POST/GET /api/notes/{id}, POST /api/notes/migrate, agent queue endpoints
- Rebuild frontend: fix duplicate loadVideos(), non-blocking localStorage migration, notes UI
- Switch pip install to Tsinghua mirror for faster downloads in China
- Add backend/sync_rotation.json to .gitignore (runtime state)

Dockerfile

@@ -16,13 +16,10 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     ffmpeg \
     && rm -rf /var/lib/apt/lists/*
 
-# Download latest official yt-dlp to ensure subscription scrapers do not break
-RUN curl -L https://github.com/yt-dlp/yt-dlp/releases/latest/download/yt-dlp -o /usr/local/bin/yt-dlp && \
-    chmod a+rx /usr/local/bin/yt-dlp
 
 # Install python packages
 COPY backend/requirements.txt ./
-RUN pip install --no-cache-dir -r requirements.txt
+RUN pip install --no-cache-dir -i https://pypi.tuna.tsinghua.edu.cn/simple -r requirements.txt
 
 # Copy built static frontend
 COPY --from=frontend-builder /app/frontend/dist ./frontend/dist