M2b：删除旧 chat 体系——线性会话/叩心/情绪/纪念日模型与页面退役（迁移 003 双向验证），runs 去会话外键，测试全量修剪

backend/app/models/runtime.py

@@ -14,13 +14,11 @@ class Run(Base):
     __tablename__ = "runs"
 
     id: Mapped[str] = mapped_column(String(80), primary_key=True, default=new_run_id)
-    # rewrite/1.0：照心试演（kind=rehearsal）无正式人物，character_id 允许为空
+    # rewrite/1.0：照心试演（kind=rehearsal）无正式人物，character_id 允许为空；
+    # 旧 chat_sessions 外键随线性会话模型退役移除
     character_id: Mapped[str | None] = mapped_column(
         String(80), ForeignKey("characters.id"), index=True, nullable=True
     )
-    session_id: Mapped[str | None] = mapped_column(
-        String(80), ForeignKey("chat_sessions.id"), index=True
-    )
     status: Mapped[str] = mapped_column(String(40), nullable=False, default="queued", index=True)
     # run.kind：foreground / background_task / awakening / rehearsal（契约词典 §8.2）
     kind: Mapped[str] = mapped_column(