✨ feat: WP-03 主体——ContentBlock 全类型、Run 事件重放、分页与分支（WP-03）

- ContentBlock 全 21 类型 Pydantic 判别联合（与 openapi 契约一一对齐，extra=forbid）；系统产生类 block（tool_call/tool_result/agent_status/approval_request/artifact）拒绝客户端伪造（契约 3.5）
- 迁移 4：run_event 表（(run_id, sequence) 唯一、事件类型检查约束）+ conversation.current_branch_id
- Run 事件持久化：状态事件、每 block completed、message.completed 与权威状态同事务写入；sequence 单调从 0；SSE 改为重放持久化事件（不再现场构造），delta/progress 留待真实模型流（WP-05），不伪造
- GET messages：opaque cursor 分页（(created_at,id) 复合键、base64 编码、limit 1-200、has_more）
- branches：列表 + 当前路径切换（采用≠归真）；跨会话/不存在分支 422 fail-closed
- 测试 36 项全绿：21 类型接受、runtime block 拒绝、事件重放序号单调、分页无重叠、cursor 非法拒绝、分支切换与跨域

backend/app/models.py

@@ -154,6 +154,9 @@ 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")
+    current_branch_id: Mapped[str | None] = mapped_column(
+        ForeignKey("branch.id", ondelete="set null", use_alter=True), nullable=True
+    )
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, server_default=func.now()
     )
@@ -249,3 +252,31 @@ class IdempotencyRecord(Base):
     __table_args__ = (
         Index("uq_idempotency_owner_route_key", "owner_domain_id", "route", "key", unique=True),
     )
+
+
+class RunEvent(Base):
+    """Run 事件流：持久化后可重放；sequence 在 Run 内单调（契约第 5 节）。"""
+
+    __tablename__ = "run_event"
+
+    id: Mapped[str] = mapped_column(String, primary_key=True, default=lambda: new_id("event_"))
+    run_id: Mapped[str] = mapped_column(
+        ForeignKey("run.id", ondelete="cascade"), nullable=False
+    )
+    sequence: Mapped[int] = mapped_column(nullable=False)
+    type: Mapped[str] = mapped_column(String, nullable=False)
+    data: Mapped[dict[str, Any]] = mapped_column(JSONB, nullable=False, default=dict)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False, server_default=func.now()
+    )
+
+    __table_args__ = (
+        CheckConstraint(
+            "type IN ('content.block.started', 'content.block.delta', 'content.block.progress', "
+            "'content.block.completed', 'content.block.failed', 'content.block.cancelled', "
+            "'message.completed', 'run.status.changed')",
+            name="type_values",
+        ),
+        # Run 内 sequence 单调且唯一（重放去重的服务端保证）
+        Index("uq_run_event_run_sequence", "run_id", "sequence", unique=True),
+    )