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/routers/chapters.py

@@ -11,17 +11,36 @@ from database import get_async_db
 from database.models import Chapter as ChapterModel
 from database.models import User as UserModel
 from middleware.auth import get_current_user
+from agents.prompts.memory_prompts import CHAPTER_CATEGORIES, CHAPTER_ORDER, STAGE_TO_ORDER
 
 router = APIRouter(prefix="/api/chapters", tags=["chapters"])
 
 
+def _chapter_to_dict(ch: ChapterModel) -> dict:
+    return {
+        "id": ch.id,
+        "title": ch.title,
+        "content": ch.content,
+        "order_index": ch.order_index,
+        "status": ch.status,
+        "category": ch.category,
+        "images": ch.images or [],
+        "updated_at": ch.updated_at.isoformat() if ch.updated_at else None,
+        "is_new": ch.is_new,
+        "source_segments": ch.source_segments or [],
+    }
+
+
 @router.get("", response_model=List[dict])
 async def get_chapters(
     current_user: UserModel = Depends(get_current_user),
     is_new: Optional[bool] = Query(None, description="仅返回未读章节"),
     db: AsyncSession = Depends(get_async_db)
 ):
-    """获取用户所有章节（需要认证，仅返回 active 章节）"""
+    """
+    获取用户所有章节（需要认证，仅返回 active 章节）。
+    始终返回全部 8 个预定义类别，没有内容的类别用占位符返回。
+    """
     stmt = select(ChapterModel).where(
         ChapterModel.user_id == current_user.id,
         ChapterModel.is_active == True
@@ -31,22 +50,37 @@ async def get_chapters(
     stmt = stmt.order_by(ChapterModel.order_index)
     result = await db.execute(stmt)
     chapters = result.scalars().all()
-    
-    return [
-        {
-            "id": ch.id,
-            "title": ch.title,
-            "content": ch.content,
-            "order_index": ch.order_index,
-            "status": ch.status,
-            "category": ch.category,
-            "images": ch.images or [],
-            "updated_at": ch.updated_at.isoformat() if ch.updated_at else None,
-            "is_new": ch.is_new,
-            "source_segments": ch.source_segments or [],
-        }
-        for ch in chapters
-    ]
+
+    chapter_by_category: dict[str, ChapterModel] = {}
+    for ch in chapters:
+        if ch.category and ch.category not in chapter_by_category:
+            chapter_by_category[ch.category] = ch
+
+    all_chapters: List[dict] = []
+    for category in CHAPTER_ORDER:
+        ch = chapter_by_category.pop(category, None)
+        if ch:
+            all_chapters.append(_chapter_to_dict(ch))
+        else:
+            if is_new is True:
+                continue
+            all_chapters.append({
+                "id": f"placeholder_{category}",
+                "title": CHAPTER_CATEGORIES[category],
+                "content": "",
+                "order_index": STAGE_TO_ORDER.get(category, 999),
+                "status": "empty",
+                "category": category,
+                "images": [],
+                "updated_at": None,
+                "is_new": False,
+                "source_segments": [],
+            })
+
+    for ch in chapter_by_category.values():
+        all_chapters.append(_chapter_to_dict(ch))
+
+    return all_chapters
 
 
 @router.get("/{chapter_id}", response_model=dict)