feat: 修正章节排序和分类逻辑

- 新增 SQL 脚本以修正章节排序索引，确保与 8 个分类体系对齐。
- 更新 API 章节获取逻辑，始终返回所有 8 个预定义类别，未填充内容的类别使用占位符。
- 引入章节分类功能，支持从 5-stage 关键词映射到 8 个章节类别，提升内容分类准确性。
- 更新 Android 客户端以适应新的章节定义和占位逻辑，确保用户界面一致性。

api/migrations/fix_chapter_order_index_v2.sql

@@ -0,0 +1,19 @@
+-- 修正章节排序索引，与 8 分类体系对齐
+-- childhood=0, education=1, career_early=2, career_achievement=3,
+-- career_challenge=4, family=5, beliefs=6, summary=7
+UPDATE chapters SET order_index = 0 WHERE category = 'childhood' AND order_index != 0;
+UPDATE chapters SET order_index = 1 WHERE category = 'education' AND order_index != 1;
+UPDATE chapters SET order_index = 2 WHERE category = 'career_early' AND order_index != 2;
+UPDATE chapters SET order_index = 3 WHERE category = 'career_achievement' AND order_index != 3;
+UPDATE chapters SET order_index = 4 WHERE category = 'career_challenge' AND order_index != 4;
+UPDATE chapters SET order_index = 5 WHERE category = 'family' AND order_index != 5;
+UPDATE chapters SET order_index = 6 WHERE category IN ('belief', 'beliefs') AND order_index != 6;
+UPDATE chapters SET order_index = 7 WHERE category = 'summary' AND order_index != 7;
+
+-- 旧的 5-stage "career" 章节归入 career_early
+UPDATE chapters SET category = 'career_early', order_index = 2
+  WHERE category = 'career';
+
+-- 旧的 "belief" 统一为 "beliefs"
+UPDATE chapters SET category = 'beliefs'
+  WHERE category = 'belief';