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/database/models.py

@@ -81,16 +81,15 @@ class Segment(Base):
 
 
 class Chapter(Base):
-    """章节表"""
+    """章节表（正文与插图存于 chapter_sections）"""
     __tablename__ = "chapters"
 
     id = Column(String, primary_key=True)
     user_id = Column(String, ForeignKey("users.id"), nullable=False)
     title = Column(String, nullable=False)
-    content = Column(Text, nullable=False)
     order_index = Column(Integer, nullable=False)
     status = Column(String, default="draft")  # draft, completed
-    images = Column(JSON, nullable=True)  # 图片元数据对象列表
+    cover_image = Column(JSON, nullable=True)  # 章节封面图（单条图片元数据）
     updated_at = Column(DateTime(timezone=True), default=utc_now, onupdate=utc_now)
     category = Column(String, nullable=True)  # 章节分类
     is_new = Column(Boolean, default=True)  # 是否为新内容（未读）
@@ -99,6 +98,72 @@ class Chapter(Base):
 
     # Relationships
     user = relationship("User", back_populates="chapters")
+    sections = relationship(
+        "ChapterSection",
+        back_populates="chapter",
+        order_by="ChapterSection.order_index",
+        cascade="all, delete-orphan",
+    )
+    images = relationship(
+        "MemoirImage",
+        back_populates="chapter",
+        foreign_keys="MemoirImage.chapter_id",
+        cascade="all, delete-orphan",
+    )
+
+
+class ChapterSection(Base):
+    """章节段落表：一章多段，每段一段正文 + 可选一张图"""
+    __tablename__ = "chapter_sections"
+
+    id = Column(String, primary_key=True)
+    chapter_id = Column(String, ForeignKey("chapters.id", ondelete="CASCADE"), nullable=False)
+    order_index = Column(Integer, nullable=False)
+    content = Column(Text, nullable=False)  # 本段正文（无占位符）
+    image_id = Column(String, ForeignKey("memoir_images.id", ondelete="SET NULL"), nullable=True)  # 关联 memoir_images.id
+    updated_at = Column(DateTime(timezone=True), default=utc_now, onupdate=utc_now)
+
+    # Relationships
+    chapter = relationship("Chapter", back_populates="sections")
+    image_record = relationship(
+        "MemoirImage",
+        back_populates="section",
+        uselist=False,
+        foreign_keys="ChapterSection.image_id",
+        cascade="all, delete-orphan",
+        single_parent=True,
+    )
+
+
+class MemoirImage(Base):
+    """章节配图与封面：字段独立存储，section_id 为空表示章节封面，非空表示该 section 的配图"""
+    __tablename__ = "memoir_images"
+
+    id = Column(String, primary_key=True)
+    chapter_id = Column(String, ForeignKey("chapters.id", ondelete="CASCADE"), nullable=False)
+    section_id = Column(String, ForeignKey("chapter_sections.id", ondelete="CASCADE"), nullable=True)
+    order_index = Column(Integer, nullable=False, default=0)
+    placeholder = Column(Text, nullable=True)
+    description = Column(Text, nullable=True)
+    status = Column(String, nullable=False, default="pending")
+    prompt = Column(Text, nullable=True)
+    url = Column(Text, nullable=True)
+    storage_key = Column(Text, nullable=True)
+    provider = Column(String, nullable=True)
+    style = Column(String, nullable=True)
+    size = Column(String, nullable=True)
+    error = Column(Text, nullable=True)
+    retryable = Column(Boolean, nullable=True)
+    created_at = Column(DateTime(timezone=True), nullable=True)
+    updated_at = Column(DateTime(timezone=True), default=utc_now, onupdate=utc_now)
+
+    # Relationships
+    chapter = relationship("Chapter", back_populates="images")
+    section = relationship(
+        "ChapterSection",
+        back_populates="image_record",
+        foreign_keys="ChapterSection.image_id",
+    )
 
 
 class Book(Base):