把“章节正文 + 图片”从 chapters 单表/JSON 结构，重构为“章节 chapter + 段落 section + 图片 memoir_images 独立表”的新数据模型，同时联动修改接口、PDF 导出、异步任务、迁移脚本、测试，以及修复 Android 端聊天列表显示问题。 (#9)

* refactor: 表结构重构，新增段落section和图片image新表

* fix: fix android app import error

* refactor: 重构文件名

* fix: 优化提示词

* fix: 消息气泡显示位置异常问题

---------

Co-authored-by: yangshilin <2157598560@qq.com>

api/services/memoir_images/serializers.py

@@ -0,0 +1,70 @@
+"""
+MemoirImage 模型与 API 用 dict 的互转（与 schema.normalize_image_asset 字段一致）。
+"""
+from datetime import datetime
+from typing import Any
+
+from database.models import MemoirImage
+
+
+def _parse_optional_datetime(s: str | None):
+    if not s:
+        return None
+    try:
+        return datetime.fromisoformat(s.replace("Z", "+00:00"))
+    except (ValueError, AttributeError):
+        return None
+
+
+def memoir_image_to_dict(m: MemoirImage | None) -> dict[str, Any] | None:
+    """将 MemoirImage 转为前端/API 使用的单条图片 dict。"""
+    if not m:
+        return None
+    d: dict[str, Any] = {
+        "placeholder": m.placeholder or "",
+        "description": m.description or "",
+        "index": m.order_index,
+        "status": m.status or "pending",
+        "prompt": m.prompt,
+        "url": m.url,
+        "storage_key": m.storage_key,
+        "provider": m.provider,
+        "style": m.style,
+        "size": m.size,
+        "error": m.error,
+        "retryable": m.retryable,
+        "created_at": m.created_at.isoformat() if m.created_at else None,
+        "updated_at": m.updated_at.isoformat() if m.updated_at else None,
+    }
+    return d
+
+
+def image_dict_to_row_kwargs(d: dict[str, Any] | None) -> dict[str, Any]:
+    """从单条图片 dict 提取可写入 MemoirImage 的字段（不含 id/chapter_id/section_id/order_index）。"""
+    if not d or not isinstance(d, dict):
+        return {}
+    created = d.get("created_at")
+    updated = d.get("updated_at")
+    if isinstance(created, str):
+        created = _parse_optional_datetime(created)
+    elif not isinstance(created, datetime):
+        created = None
+    if isinstance(updated, str):
+        updated = _parse_optional_datetime(updated)
+    elif not isinstance(updated, datetime):
+        updated = None
+    return {
+        "placeholder": d.get("placeholder") or None,
+        "description": d.get("description") or None,
+        "status": (d.get("status") or "pending").strip() or "pending",
+        "prompt": d.get("prompt") or None,
+        "url": d.get("url") or None,
+        "storage_key": d.get("storage_key") or None,
+        "provider": d.get("provider") or None,
+        "style": d.get("style") or None,
+        "size": d.get("size") or None,
+        "error": d.get("error") or None,
+        "retryable": d.get("retryable") if d.get("retryable") is not None else None,
+        "created_at": created,
+        "updated_at": updated,
+    }