feat(backend): AI 对话聊天记录 repo + append/list 端点（AC-2）

在 C2 扩(AC-1) `ai_messages` 表上落 AC-2 后端（计划 §3）：
- repo `SqlAiMessageRepo`：批 append（seq=批内 0 基位置、同 thread_id、只 flush）
  + `list_for_chapter` newest-first（created_at DESC, seq DESC）、chapter_no 过滤含
  项目级 NULL union、可选 kind、limit/offset。
- schemas `ai_messages.py`：批级 append 信封 + Literal 枚举权威（kind/role）+ content
  200k 上限 + meta 序列化兜底。
- 端点 `POST/GET /projects/{id}/ai-messages`（tag ai-messages，已注册）：显式记录写入，
  五个生成端点保持只读不变（守 #3）；项目 404、校验 422、POST 端点提交。
- 注入缝 `get_ai_message_repo`。

TDD：repo 真 pg 单测（seq/定序/union/分页/flush-only）9 passed；端点集成测试
（round-trip/union/kind/404/422×5/commit 边界）12 passed。门禁全绿：ruff/format/mypy
238 files/alembic check 无漂移/pytest 992 passed；新模块 cov router+schemas 100% / repo 97%。

apps/api/ww_api/routers/ai_messages.py

@@ -0,0 +1,114 @@
+"""AI 对话（聊天记录）端点（AC-2 / ai-chat-history-plan §3.3）。
+
+作者↔AI 五类往复交换（refine/rewrite/clarify/continue/generator）落库，供每章「AI 对话」
+抽屉重放。**本端点是显式的记录写入**（前端在一轮交换结束后 POST 一批）——不是生成端点；
+五个生成端点保持只读、逐字节不变（守不变量 #3）。`ai_messages` 是侧记录，assemble()/agent
+永不读它（守 #1/#6）。
+
+- POST /projects/{id}/ai-messages   一批 append（201；项目不存在 → 404）。
+- GET  /projects/{id}/ai-messages    列出（newest-first；`chapter_no` 过滤含项目级 NULL；分页）。
+
+提交边界：`AiMessageRepo.append` 只 flush，POST 端点写后 `commit()`（仿模板/规则写侧）。
+GET 只读不 commit。
+"""
+
+from __future__ import annotations
+
+import uuid
+from typing import Annotated
+
+from fastapi import APIRouter, Depends, Query, Request
+from sqlalchemy.ext.asyncio import AsyncSession
+from ww_core.domain import AiMessageRepo, AiTurnRow, ProjectRepo
+from ww_core.domain.ai_message_repo import AiMessageView as AiMessageRepoView
+from ww_db import get_session
+from ww_shared import AppError, ErrorCode
+
+from ww_api.logging_config import get_logger
+from ww_api.pagination import PageDep
+from ww_api.schemas.ai_messages import (
+    AiKind,
+    AiMessageAppendRequest,
+    AiMessageListResponse,
+    AiMessageView,
+)
+from ww_api.services.credentials import STUB_OWNER_ID
+from ww_api.services.project_deps import get_ai_message_repo, get_project_repo
+
+log = get_logger("ww.api.ai_messages")
+
+router = APIRouter(prefix="/projects", tags=["ai-messages"])
+
+AiMessageRepoDep = Annotated[AiMessageRepo, Depends(get_ai_message_repo)]
+ProjectRepoDep = Annotated[ProjectRepo, Depends(get_project_repo)]
+SessionDep = Annotated[AsyncSession, Depends(get_session)]
+
+
+def _to_response_view(v: AiMessageRepoView) -> AiMessageView:
+    """repo 视图 → API 视图（同字段镜像；避免路由碰 repo 视图类型泄漏）。"""
+    return AiMessageView(
+        id=v.id,
+        project_id=v.project_id,
+        chapter_no=v.chapter_no,
+        thread_id=v.thread_id,
+        seq=v.seq,
+        kind=v.kind,
+        tool_key=v.tool_key,
+        role=v.role,
+        content=v.content,
+        meta=v.meta,
+        created_at=v.created_at,
+    )
+
+
+@router.post("/{project_id}/ai-messages", status_code=201)
+async def append_ai_messages(
+    project_id: uuid.UUID,
+    body: AiMessageAppendRequest,
+    request: Request,
+    project_repo: ProjectRepoDep,
+    repo: AiMessageRepoDep,
+    session: SessionDep,
+) -> AiMessageListResponse:
+    """一批往复 bubble append（201）。项目不存在 → 404；非法 kind/role/空/超上限 → 422。"""
+    request_id = getattr(request.state, "request_id", None)
+    project = await project_repo.get(STUB_OWNER_ID, project_id)
+    if project is None:
+        raise AppError(ErrorCode.NOT_FOUND, f"project not found: {project_id}")
+    views = await repo.append(
+        project_id,
+        thread_id=body.thread_id,
+        chapter_no=body.chapter_no,
+        kind=body.kind,
+        tool_key=body.tool_key,
+        rows=[AiTurnRow(role=m.role, content=m.content, meta=m.meta) for m in body.messages],
+    )
+    await session.commit()
+    log.info(
+        "ai_messages_appended",
+        project_id=str(project_id),
+        request_id=request_id,
+        thread_id=str(body.thread_id),
+        kind=body.kind,
+        count=len(views),
+    )
+    return AiMessageListResponse(messages=[_to_response_view(v) for v in views])
+
+
+@router.get("/{project_id}/ai-messages")
+async def list_ai_messages(
+    project_id: uuid.UUID,
+    project_repo: ProjectRepoDep,
+    repo: AiMessageRepoDep,
+    page: PageDep,
+    chapter_no: Annotated[int | None, Query(ge=1)] = None,
+    kind: AiKind | None = None,
+) -> AiMessageListResponse:
+    """列出 AI 对话（newest-first）。`chapter_no` → 本章 ∪ 项目级(NULL)；缺省 → 仅项目级。"""
+    project = await project_repo.get(STUB_OWNER_ID, project_id)
+    if project is None:
+        raise AppError(ErrorCode.NOT_FOUND, f"project not found: {project_id}")
+    views = await repo.list_for_chapter(
+        project_id, chapter_no, kind=kind, limit=page.limit, offset=page.offset
+    )
+    return AiMessageListResponse(messages=[_to_response_view(v) for v in views])