✨ feat: WP-03 三界归真作用域与消息版本谱系（ADR-020 落地）

- 迁移 5：conversation.realm（默认真界，检查约束）；message.realm 快照、canonized_at、parent_message_id 自引用与 variant_kind（origin/revision/candidate）
- 界域端点：GET conversation（realm/当前分支）与 POST realm（入境/出境显式切换，留审计——ADR-020 切换必须可见）
- 归真端点 POST /v1/messages/{id}/canonizations：scope 必须与消息界域一致（不一致 422）；幂等（重复归真 200 原状态）；境界归真产出 world.canonized Outbox 事件（世界知识候选入口，WP-04 消费）；幻境归真仅标记试演记录
- 编辑版本 POST /v1/messages/{id}/revisions：新消息 variant=revision 引用原消息（markdown 修订同样经灵台引擎规范化），原消息永不修改
- 测试 44 项全绿（+8）：界域快照、归真正/反/幂等、幻境试演、revision 原消息不变、跨域 401/403/404 fail-closed

backend/app/models.py

@@ -142,6 +142,11 @@ class OutboxEvent(Base):
     )
 
 
+CONVERSATION_REALM_CK = CheckConstraint(
+    "realm IN ('reality', 'world', 'simulation')", name="realm_values"
+)
+
+
 class Conversation(Base):
     """对话容器：归属个人域（垂直切片 WP-03 最小集）。"""
 
@@ -154,6 +159,7 @@ class Conversation(Base):
         ForeignKey("personal_domain.id", ondelete="cascade"), nullable=False, index=True
     )
     title: Mapped[str] = mapped_column(String, nullable=False, default="未命名对话")
+    realm: Mapped[str] = mapped_column(String, nullable=False, default="reality")
     current_branch_id: Mapped[str | None] = mapped_column(
         ForeignKey("branch.id", ondelete="set null", use_alter=True), nullable=True
     )
@@ -164,6 +170,8 @@ class Conversation(Base):
         DateTime(timezone=True), nullable=False, server_default=func.now(), onupdate=func.now()
     )
 
+    __table_args__ = (CONVERSATION_REALM_CK,)
+
 
 class Branch(Base):
     """对话分支：创建会话时生成默认主分支。"""
@@ -193,6 +201,12 @@ class Message(Base):
         ForeignKey("branch.id", ondelete="cascade"), nullable=False, index=True
     )
     role: Mapped[str] = mapped_column(String, nullable=False)
+    realm: Mapped[str] = mapped_column(String, nullable=False, default="reality")
+    canonized_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
+    parent_message_id: Mapped[str | None] = mapped_column(
+        ForeignKey("message.id", ondelete="set null"), nullable=True
+    )
+    variant_kind: Mapped[str] = mapped_column(String, nullable=False, default="origin")
     content: Mapped[list[dict[str, Any]]] = mapped_column(JSONB, nullable=False)
     parser_version: Mapped[str | None] = mapped_column(String, nullable=True)
     created_at: Mapped[datetime] = mapped_column(
@@ -201,6 +215,12 @@ class Message(Base):
 
     __table_args__ = (
         CheckConstraint("role IN ('user', 'assistant', 'system', 'tool')", name="role_values"),
+        CheckConstraint(
+            "realm IN ('reality', 'world', 'simulation')", name="realm_values"
+        ),
+        CheckConstraint(
+            "variant_kind IN ('origin', 'revision', 'candidate')", name="variant_kind_values"
+        ),
     )