✨ 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/conversations/routes.py

@@ -102,7 +102,7 @@ async def run_events(
     db: DbSession,
     user: Annotated[User, Depends(get_current_user)],
 ) -> StreamingResponse:
-    """Run SSE：返回当前快照与终止状态（垂直切片空状态闭环）。"""
+    """Run SSE：重放持久化事件流（sequence 单调），客户端按序去重（契约第 5 节）。"""
     result = await db.execute(select(Run).where(Run.id == run_id))
     run = result.scalar_one_or_none()
     if run is None:
@@ -111,29 +111,98 @@ async def run_events(
     if run.personal_domain_id != domain.id:
         raise HTTPException(status.HTTP_404_NOT_FOUND, "Run 不存在")
 
-    terminal = run.status in {"completed", "failed", "cancelled"}
+    from app.models import RunEvent
+
+    events = (
+        await db.execute(
+            select(RunEvent)
+            .where(RunEvent.run_id == run.id)
+            .order_by(RunEvent.sequence.asc())
+        )
+    ).scalars().all()
 
     async def event_stream() -> AsyncIterator[str]:
-        def envelope(event_type: str, data: dict[str, object], sequence: int) -> str:
+        for event in events:
             payload = json.dumps(
-                {"type": event_type, "data": data, "sequence": sequence}, ensure_ascii=False
-            )
-            return f"data: {payload}\n\n"
-
-        yield envelope("run.status.changed", {"run_id": run.id, "status": run.status}, 0)
-        if terminal:
-            yield envelope(
-                "message.completed",
-                {
-                    "run_id": run.id,
-                    "message_ids": [],
-                    "final_status": run.status,
-                },
-                1,
+                {"type": event.type, "data": event.data, "sequence": event.sequence},
+                ensure_ascii=False,
             )
+            yield f"data: {payload}\n\n"
 
     return StreamingResponse(
         event_stream(),
         media_type="text/event-stream",
         headers={"Cache-Control": "no-store"},
     )
+
+
+@router.get("/{conversation_id}/messages")
+async def list_messages(
+    conversation_id: str,
+    db: DbSession,
+    user: Annotated[User, Depends(get_current_user)],
+    limit: int = 50,
+    cursor: str | None = None,
+    branch_id: str | None = None,
+) -> dict[str, object]:
+    """消息列表：opaque cursor 分页（默认 50，契约第 4.1 节）。"""
+    if not 1 <= limit <= 200:
+        raise HTTPException(status.HTTP_422_UNPROCESSABLE_ENTITY, "limit 取值范围 1-200")
+    domain = await _domain_of(db, user)
+    page, next_cursor, has_more = await service.list_messages(
+        db, domain.id, conversation_id, branch_id, limit, cursor
+    )
+    from app.conversations.schemas import MessageOut
+
+    return {
+        "data": [
+            MessageOut(
+                id=m.id,
+                conversation_id=m.conversation_id,
+                branch_id=m.branch_id,
+                role=m.role,
+                content=m.content,
+                parser_version=m.parser_version,
+                created_at=m.created_at.isoformat(),
+            ).model_dump()
+            for m in page
+        ],
+        "next_cursor": next_cursor,
+        "has_more": has_more,
+    }
+
+
+@router.get("/{conversation_id}/branches")
+async def list_branches(
+    conversation_id: str,
+    db: DbSession,
+    user: Annotated[User, Depends(get_current_user)],
+) -> dict[str, object]:
+    domain = await _domain_of(db, user)
+    branches = await service.list_branches(db, domain.id, conversation_id)
+    from app.conversations.schemas import BranchOut
+
+    return {
+        "data": [
+            BranchOut(
+                id=b.id, conversation_id=b.conversation_id, title=b.title,
+                created_at=b.created_at.isoformat(),
+            ).model_dump()
+            for b in branches
+        ]
+    }
+
+
+@router.post("/{conversation_id}/current-branch")
+async def set_current_branch(
+    conversation_id: str,
+    db: DbSession,
+    user: Annotated[User, Depends(require_state_change)],
+    body: dict[str, str],
+) -> dict[str, str]:
+    """切换当前路径（主脉工作入口；采用≠归真）。"""
+    domain = await _domain_of(db, user)
+    conversation = await service.set_current_branch(
+        db, domain.id, conversation_id, body.get("branch_id", "")
+    )
+    return {"id": conversation.id, "current_branch_id": conversation.current_branch_id or ""}