feat: Add cross-sectional strategy support

- Add cross-sectional strategy type (single vs cross-sectional)
- Support multi-symbol portfolio management with automatic ranking
- Add portfolio size, long ratio, and rebalance frequency configuration
- Implement parallel order execution for cross-sectional strategies
- Add frontend UI for strategy type selection and configuration
- Add i18n support (Chinese and English) for cross-sectional features
- Fix decimal precision issues in exchange order quantities
- Add last_rebalance_at field to database schema
- Add comprehensive documentation and examples

Database migration required: Add last_rebalance_at column to qd_strategies_trading table

backend_api_python/migrations/init.sql

@@ -162,6 +162,18 @@ CREATE INDEX IF NOT EXISTS idx_strategies_user_id ON qd_strategies_trading(user_
 CREATE INDEX IF NOT EXISTS idx_strategies_status ON qd_strategies_trading(status);
 CREATE INDEX IF NOT EXISTS idx_strategies_group_id ON qd_strategies_trading(strategy_group_id);
 
+-- Add last_rebalance_at column for cross-sectional strategies (if not exists)
+DO $$
+BEGIN
+    IF NOT EXISTS (
+        SELECT 1 FROM information_schema.columns 
+        WHERE table_name = 'qd_strategies_trading' AND column_name = 'last_rebalance_at'
+    ) THEN
+        ALTER TABLE qd_strategies_trading ADD COLUMN last_rebalance_at TIMESTAMP;
+        RAISE NOTICE 'Added last_rebalance_at column to qd_strategies_trading';
+    END IF;
+END $$;
+
 -- =============================================================================
 -- 3. Strategy Positions
 -- =============================================================================