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/migrations/add_chapter_sections.sql

@@ -0,0 +1,26 @@
+-- 章节拆分为 chapter_sections：每段正文 + 配图独立存储，chapters 只保留封面图
+-- 执行顺序: 1) 本文件  2) python -m scripts.migrate_chapters_to_sections
+-- 执行方式: psql -U <user> -d <database> -f api/migrations/add_chapter_sections.sql
+
+-- ========== 1. 新建 chapter_sections 表 ==========
+CREATE TABLE IF NOT EXISTS chapter_sections (
+    id VARCHAR NOT NULL PRIMARY KEY,
+    chapter_id VARCHAR NOT NULL REFERENCES chapters(id) ON DELETE CASCADE,
+    order_index INTEGER NOT NULL,
+    content TEXT NOT NULL,
+    image JSONB,
+    updated_at TIMESTAMP WITH TIME ZONE DEFAULT NOW()
+);
+CREATE INDEX IF NOT EXISTS ix_chapter_sections_chapter_id ON chapter_sections(chapter_id);
+CREATE INDEX IF NOT EXISTS ix_chapter_sections_order ON chapter_sections(chapter_id, order_index);
+
+-- ========== 2. chapters 表增加 cover_image ==========
+DO $$
+BEGIN
+    IF NOT EXISTS (SELECT 1 FROM information_schema.columns WHERE table_schema = 'public' AND table_name = 'chapters' AND column_name = 'cover_image') THEN
+        ALTER TABLE chapters ADD COLUMN cover_image JSONB;
+        RAISE NOTICE '已添加 chapters.cover_image';
+    END IF;
+END $$;
+
+-- ========== 3. 回填与删列由脚本 scripts.migrate_chapters_to_sections 完成 ==========

api/migrations/add_memoir_images_table.sql

@@ -0,0 +1,32 @@
+-- 图片独立表：将原先挤在 chapter.cover_image / chapter_sections.image 的 JSON 拆成独立列
+-- 执行顺序: 1) 本文件  2) python -m api.scripts.run_memoir_images_migration
+-- 执行方式: psql -U <user> -d <database> -f api/migrations/add_memoir_images_table.sql
+
+-- ========== 1. 新建 memoir_images 表 ==========
+CREATE TABLE IF NOT EXISTS memoir_images (
+    id VARCHAR NOT NULL PRIMARY KEY,
+    chapter_id VARCHAR NOT NULL REFERENCES chapters(id) ON DELETE CASCADE,
+    section_id VARCHAR NULL REFERENCES chapter_sections(id) ON DELETE CASCADE,
+    order_index INTEGER NOT NULL DEFAULT 0,
+    placeholder TEXT,
+    description TEXT,
+    status VARCHAR NOT NULL DEFAULT 'pending',
+    prompt TEXT,
+    url TEXT,
+    storage_key TEXT,
+    provider VARCHAR,
+    style VARCHAR,
+    size VARCHAR,
+    error TEXT,
+    retryable BOOLEAN,
+    created_at TIMESTAMP WITH TIME ZONE,
+    updated_at TIMESTAMP WITH TIME ZONE DEFAULT NOW()
+);
+
+COMMENT ON TABLE memoir_images IS '章节配图与封面：section_id 为空表示章节封面，非空表示该 section 的配图';
+COMMENT ON COLUMN memoir_images.order_index IS '章节内唯一：封面=0，段落配图=1,2,3,...（对应 section.order_index+1）';
+
+CREATE INDEX IF NOT EXISTS ix_memoir_images_chapter_id ON memoir_images(chapter_id);
+CREATE INDEX IF NOT EXISTS ix_memoir_images_section_id ON memoir_images(section_id);
+CREATE UNIQUE INDEX IF NOT EXISTS ix_memoir_images_chapter_cover ON memoir_images(chapter_id) WHERE section_id IS NULL;
+CREATE UNIQUE INDEX IF NOT EXISTS ix_memoir_images_section_unique ON memoir_images(section_id) WHERE section_id IS NOT NULL;

api/migrations/add_section_image_id_fk.sql

@@ -0,0 +1,18 @@
+-- section 表用 image_id 关联 memoir_images，不再存 JSON
+-- 执行顺序：1) 本文件  2) 回填后执行 DROP 语句（或由脚本完成）
+-- 执行方式: psql -U <user> -d <database> -f api/migrations/add_section_image_id_fk.sql
+
+-- 1. 添加外键列（可空，无默认）
+ALTER TABLE chapter_sections
+ADD COLUMN IF NOT EXISTS image_id VARCHAR REFERENCES memoir_images(id) ON DELETE SET NULL;
+
+-- 2. 回填：已有 memoir_images 且 section_id 指向本行的，把其 id 写入本行 image_id
+UPDATE chapter_sections cs
+SET image_id = sub.id
+FROM (
+    SELECT id, section_id FROM memoir_images WHERE section_id IS NOT NULL
+) sub
+WHERE sub.section_id = cs.id AND cs.image_id IS NULL;
+
+-- 3. 删除旧的 JSON 列
+ALTER TABLE chapter_sections DROP COLUMN IF EXISTS image;

api/migrations/fix_memoir_images_order_index.sql

@@ -0,0 +1,9 @@
+-- 修复 memoir_images 同一章节内 order_index 重复：封面=0，段落配图=1,2,3,...（section.order_index+1）
+-- 仅更新有 section_id 的段落配图，封面（section_id 为空）保持 0。
+-- 执行方式: psql -U <user> -d <database> -f api/migrations/fix_memoir_images_order_index.sql
+
+UPDATE memoir_images mi
+SET order_index = cs.order_index + 1
+FROM chapter_sections cs
+WHERE mi.section_id IS NOT NULL
+  AND mi.section_id = cs.id;