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)

backend/db.py

@@ -23,6 +23,25 @@ async def init_db():
                 created_at TEXT DEFAULT CURRENT_TIMESTAMP
             )
         """)
+        await db.execute("""
+            CREATE TABLE IF NOT EXISTS video_notes (
+                video_id TEXT PRIMARY KEY,
+                note_text TEXT NOT NULL,
+                updated_at TEXT DEFAULT CURRENT_TIMESTAMP,
+                FOREIGN KEY (video_id) REFERENCES videos(video_id)
+            )
+        """)
+        await db.execute("""
+            CREATE TABLE IF NOT EXISTS agent_queue (
+                id INTEGER PRIMARY KEY AUTOINCREMENT,
+                video_id TEXT NOT NULL,
+                user_prompt TEXT NOT NULL,
+                status TEXT DEFAULT 'pending',
+                created_at TEXT DEFAULT CURRENT_TIMESTAMP,
+                processed_at TEXT,
+                FOREIGN KEY (video_id) REFERENCES videos(video_id)
+            )
+        """)
         await db.commit()
 
 async def get_db():