✨ 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/service.py

@@ -13,7 +13,7 @@ from fastapi import HTTPException, status
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
-from app.conversations.schemas import MessageIn
+from app.conversations.schemas import RUNTIME_BLOCK_TYPES, MessageIn
 from app.core import lingtai
 from app.core.ids import new_id
 from app.models import (
@@ -24,6 +24,7 @@ from app.models import (
     Message,
     OutboxEvent,
     Run,
+    RunEvent,
 )
 
 MESSAGE_ROUTE = "/v1/conversations/{conversation_id}/messages"
@@ -123,6 +124,13 @@ async def replay_or_execute_message(
     conversation = await _require_conversation(db, domain_id, conversation_id)
     branch = await _get_default_branch(db, conversation.id)
 
+    forged = [b.type for b in body.content if b.type in RUNTIME_BLOCK_TYPES]
+    if forged:
+        raise HTTPException(
+            status.HTTP_422_UNPROCESSABLE_ENTITY,
+            f"系统产生类 block 不允许客户端提交：{', '.join(sorted(set(forged)))}",
+        )
+
     blocks: list[dict[str, Any]] = []
     parser_version: str | None = None
     for b in body.content:
@@ -161,6 +169,7 @@ async def replay_or_execute_message(
     resource_ids: dict[str, Any] = {"message_id": message.id}
     if run is not None:
         resource_ids["run_id"] = run.id
+        _record_run_events(db, run, message, blocks)
     db.add(
         AuditEvent(
             actor_type="user",
@@ -191,3 +200,105 @@ async def replay_or_execute_message(
     )
     await db.flush()
     return message, run, False
+
+
+def _record_run_events(
+    db: AsyncSession, run: Run, message: Message, blocks: list[dict[str, Any]]
+) -> None:
+    """空状态闭环的事件流：状态事件 + 每 block completed + message.completed。
+
+    sequence 在 Run 内从 0 单调；事件与权威状态同事务提交（契约第 5 节）。
+    delta/progress 事件在模型流式接入（WP-05）后由真实流产生，此处不伪造。
+    """
+    seq = 0
+    events: list[tuple[str, dict[str, Any]]] = [
+        ("run.status.changed", {"run_id": run.id, "status": run.status})
+    ]
+    for index, block in enumerate(blocks):
+        events.append(
+            (
+                "content.block.completed",
+                {"run_id": run.id, "message_id": message.id, "index": index,
+                 "block": {"id": block["id"], "type": block["type"], "status": "completed"}},
+            )
+        )
+    events.append(
+        (
+            "message.completed",
+            {"run_id": run.id, "message_ids": [message.id], "final_status": run.status},
+        )
+    )
+    for event_type, data in events:
+        db.add(RunEvent(run_id=run.id, sequence=seq, type=event_type, data=data))
+        seq += 1
+
+
+async def list_messages(
+    db: AsyncSession,
+    domain_id: str,
+    conversation_id: str,
+    branch_id: str | None,
+    limit: int,
+    cursor: str | None,
+) -> tuple[list[Message], str | None, bool]:
+    """按创建时间升序的消息列表（opaque cursor 分页，契约第 4.1 节）。"""
+    conversation = await _require_conversation(db, domain_id, conversation_id)
+    stmt = select(Message).where(Message.conversation_id == conversation.id)
+    if branch_id is not None:
+        stmt = stmt.where(Message.branch_id == branch_id)
+    after_created, after_id = decode_cursor(cursor)
+    if after_created is not None:
+        # (created_at, id) 复合键分页：严格大于游标位置
+        stmt = stmt.where(
+            (Message.created_at > after_created)
+            | ((Message.created_at == after_created) & (Message.id > after_id))
+        )
+    stmt = stmt.order_by(Message.created_at.asc(), Message.id.asc()).limit(limit + 1)
+    rows = list((await db.scalars(stmt)).all())
+    has_more = len(rows) > limit
+    page = rows[:limit]
+    next_cursor = encode_cursor(page[-1]) if has_more and page else None
+    return page, next_cursor, has_more
+
+
+def encode_cursor(message: Message) -> str:
+    import base64
+
+    raw = f"{message.created_at.isoformat()}|{message.id}"
+    return base64.urlsafe_b64encode(raw.encode()).decode()
+
+
+def decode_cursor(cursor: str | None) -> tuple[Any | None, str]:
+    import base64
+
+    if cursor is None:
+        return None, ""
+    try:
+        raw = base64.urlsafe_b64decode(cursor.encode()).decode()
+        created_str, _, mid = raw.partition("|")
+        return datetime.fromisoformat(created_str), mid
+    except Exception as exc:
+        raise HTTPException(status.HTTP_422_UNPROCESSABLE_ENTITY, "cursor 不合法") from exc
+
+
+async def list_branches(
+    db: AsyncSession, domain_id: str, conversation_id: str
+) -> list[Branch]:
+    conversation = await _require_conversation(db, domain_id, conversation_id)
+    result = await db.execute(
+        select(Branch).where(Branch.conversation_id == conversation.id).order_by(Branch.created_at)
+    )
+    return list(result.scalars().all())
+
+
+async def set_current_branch(
+    db: AsyncSession, domain_id: str, conversation_id: str, branch_id: str
+) -> Conversation:
+    """切换当前路径：分支必须属于该会话（跨域/跨会话统一 404/422 fail-closed）。"""
+    conversation = await _require_conversation(db, domain_id, conversation_id)
+    branch = await db.get(Branch, branch_id)
+    if branch is None or branch.conversation_id != conversation.id:
+        raise HTTPException(status.HTTP_422_UNPROCESSABLE_ENTITY, "分支不属于该会话")
+    conversation.current_branch_id = branch_id
+    await db.flush()
+    return conversation