feat(pl): add pl_plan.copy_seq_no for copy-to-group sequence

Persist a tenant-scoped fixed serial for driver-plan clipboard text;
include migration backfill by create_time.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/models/pl_models.py

@@ -274,6 +274,7 @@ class PlPlan(Base):
 
     # 编号信息
     plan_no = Column(String(32), nullable=False, unique=True, comment="计划订单编号 POYYYYMMDD###")
+    copy_seq_no = Column(Integer, nullable=True, comment="复制发群序号（租户内固定流水）")
     create_date = Column(Date, nullable=False, comment="创建日期")
     plan_date = Column(Date, nullable=True, comment="计划日期")