新增作文区与 AI 解读开关，修复 CSV 导出。

系统设置可关闭成绩复盘 AI；学生详情增加作文区（OCR/手动题目、方案与范文、历史与 MD 下载）；导出改用 UTF-8 文件名响应。

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/schemas/__init__.py

@@ -76,8 +76,13 @@ class PublicSettingsOut(BaseModel):
     registration_enabled: bool
 
 
+class AppFeaturesOut(BaseModel):
+    ai_review_enabled: bool
+
+
 class SystemSettingsOut(BaseModel):
     registration_enabled: bool
+    ai_review_enabled: bool = True
     ai_provider: AIProviderEnum
     ollama_base_url: str | None = None
     ollama_model: str | None = None
@@ -92,6 +97,7 @@ class SystemSettingsOut(BaseModel):
 
 class SystemSettingsUpdate(BaseModel):
     registration_enabled: bool | None = None
+    ai_review_enabled: bool | None = None
     ai_provider: AIProviderEnum | None = None
     ollama_base_url: str | None = None
     ollama_model: str | None = None
@@ -293,3 +299,39 @@ class WrongQuestionUpdate(BaseModel):
     solution_approach: str | None = None
     solution_text: str | None = None
     subject_id: int | None = None
+
+
+class CompositionStatusEnum(str, Enum):
+    pending = "pending"
+    generating = "generating"
+    done = "done"
+    failed = "failed"
+
+
+class CompositionInputModeEnum(str, Enum):
+    manual = "manual"
+    ocr = "ocr"
+
+
+class CompositionCreate(BaseModel):
+    topic: str = Field(..., min_length=1, max_length=4000)
+    input_mode: CompositionInputModeEnum = CompositionInputModeEnum.manual
+
+
+class CompositionOcrOut(BaseModel):
+    text: str
+
+
+class CompositionOut(BaseModel):
+    id: UUID
+    student_id: UUID
+    topic: str
+    input_mode: CompositionInputModeEnum
+    writing_plan: str | None = None
+    sample_essay: str | None = None
+    error_message: str | None = None
+    status: CompositionStatusEnum
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = {"from_attributes": True}