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

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

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;