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/models/user.py

@@ -27,6 +27,18 @@ class WrongQuestionCategory(str, enum.Enum):
     olympiad = "olympiad"
 
 
+class CompositionStatus(str, enum.Enum):
+    pending = "pending"
+    generating = "generating"
+    done = "done"
+    failed = "failed"
+
+
+class CompositionInputMode(str, enum.Enum):
+    manual = "manual"
+    ocr = "ocr"
+
+
 class AIProvider(str, enum.Enum):
     ollama = "ollama"
     openai = "openai"
@@ -73,6 +85,9 @@ class Student(Base):
     wrong_questions: Mapped[list["WrongQuestion"]] = relationship(
         back_populates="student", cascade="all, delete-orphan"
     )
+    compositions: Mapped[list["Composition"]] = relationship(
+        back_populates="student", cascade="all, delete-orphan"
+    )
 
 
 class Subject(Base):
@@ -150,11 +165,37 @@ class WrongQuestion(Base):
     subject: Mapped["Subject"] = relationship(back_populates="wrong_questions")
 
 
+class Composition(Base):
+    __tablename__ = "compositions"
+
+    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    student_id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), ForeignKey("students.id"), index=True)
+    topic: Mapped[str] = mapped_column(Text)
+    input_mode: Mapped[CompositionInputMode] = mapped_column(
+        Enum(CompositionInputMode), default=CompositionInputMode.manual
+    )
+    writing_plan: Mapped[str | None] = mapped_column(Text, nullable=True)
+    sample_essay: Mapped[str | None] = mapped_column(Text, nullable=True)
+    error_message: Mapped[str | None] = mapped_column(Text, nullable=True)
+    status: Mapped[CompositionStatus] = mapped_column(
+        Enum(CompositionStatus), default=CompositionStatus.pending
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
+    )
+
+    student: Mapped["Student"] = relationship(back_populates="compositions")
+
+
 class SystemSettings(Base):
     __tablename__ = "system_settings"
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True, default=1)
     registration_enabled: Mapped[bool] = mapped_column(default=True)
+    ai_review_enabled: Mapped[bool] = mapped_column(default=True)
     ai_provider: Mapped[str] = mapped_column(String(16), default="ollama")
     ollama_base_url: Mapped[str | None] = mapped_column(String(256), nullable=True)
     ollama_model: Mapped[str | None] = mapped_column(String(128), nullable=True)