feature: 新增支持拆书自选保留章节数，或者整本拆书功能

backend/app/schemas/book_import.py

@@ -9,6 +9,7 @@ TaskStatus = Literal["pending", "running", "completed", "failed", "cancelled"]
 ImportMode = Literal["append", "overwrite"]
 ExtractLevel = Literal["basic", "standard", "deep"]
 WarningLevel = Literal["info", "warning", "error"]
+BookImportExtractMode = Literal["tail", "full"]
 
 
 class BookImportWarning(BaseModel):
@@ -45,6 +46,12 @@ class BookImportOutline(BaseModel):
     structure: Optional[dict[str, Any]] = Field(None, description="结构化大纲（与系统大纲生成结构一致）")
 
 
+class BookImportTaskCreateRequest(BaseModel):
+    """创建拆书任务请求"""
+    extract_mode: BookImportExtractMode = Field(default="tail", description="提取范围：tail=截取末章，full=整本")
+    tail_chapter_count: int = Field(default=10, ge=5, le=9999, description="当 extract_mode=tail 时，截取末尾章节数；需为5的倍数，超过50将按整本处理")
+
+
 class BookImportTaskCreateResponse(BaseModel):
     """创建任务响应"""
     task_id: str
@@ -89,4 +96,4 @@ class BookImportApplyResponse(BaseModel):
 
 class BookImportRetryRequest(BaseModel):
     """重试失败步骤请求"""
-    steps: list[str] = Field(..., min_length=1, description="需要重试的步骤名列表，如 world_building / career_system / characters")
+    steps: list[str] = Field(..., min_length=1, description="需要重试的步骤名列表，如 world_building / career_system / characters")