fix: 删书即删对话——清理已删除书籍遗留的孤儿对话

根因：Conversation 与 Book 无 ORM 级联关系，且 SQLite 外键未强制开启，
delete_book 只删书/章节/段落，书的对话变成孤儿残留（book_id 指向不存在的书），
前端确认文案「书、目录和对话都会被删除」与事实不符，AI 助手会把这些幽灵
对话当作阅读记录。

- books.py delete_book: 显式删除该书的全部对话（消息经 ORM cascade 一并删）
- database.py _migrate: 启动时清理孤儿对话及其消息（book_id 不在 books 表内；
  assistant 全局对话 book_id 为空不受影响），幂等，本地/服务器下次启动即生效

Co-Authored-By: Claude Code <noreply@anthropic.com>

backend/app/api/books.py

@@ -10,6 +10,7 @@ from sqlalchemy import select, func
 from app.core.database import get_db, async_session
 from app.core.config import settings
 from app.models.book import Book, Chapter, Segment
+from app.models.chat import Conversation
 from app.schemas.book import BookOut, BookDetailOut, ChapterOut, SegmentOut
 from app.epub.parser import parse_epub
 from app.rag.vector import ensure_embeddings
@@ -139,8 +140,16 @@ async def delete_book(book_id: str, db: AsyncSession = Depends(get_db)):
     if not book:
         raise HTTPException(404, "书不存在")
     file_path = book.file_path
+    # 删书即删对话（与前端确认文案「书、目录和对话都会被删除」一致）。
+    # Conversation 与 Book 没有 ORM 级联关系、SQLite 外键又未强制开启，
+    # 必须显式删除该书的对话；其消息经 Conversation.messages 的 cascade 一并删除。
+    convs = (await db.execute(
+        select(Conversation).where(Conversation.book_id == book_id)
+    )).scalars().all()
+    for c in convs:
+        await db.delete(c)
     await db.delete(book)
     await db.commit()
     if file_path and os.path.exists(file_path):
         os.remove(file_path)
-    return {"deleted": book_id}
+    return {"deleted": book_id, "conversations": len(convs)}

backend/app/core/database.py

@@ -53,6 +53,15 @@ async def _migrate():
             )
             await conn.exec_driver_sql("DROP TABLE conversations_old")
 
+        # 数据完整性：删书即删对话。清理由已删除书籍遗留的孤儿对话及其消息
+        #（book_id 指向不存在的书；assistant 全局对话 book_id 为空，不受影响）。
+        await conn.exec_driver_sql(
+            "DELETE FROM conversations WHERE book_id IS NOT NULL AND book_id NOT IN (SELECT id FROM books)"
+        )
+        await conn.exec_driver_sql(
+            "DELETE FROM messages WHERE conversation_id NOT IN (SELECT id FROM conversations)"
+        )
+
 
 async def get_db():
     async with async_session() as session: