迁移合同费用通用附件

1、删除旧费用附件 API、CRUD、模型和专用前端面板，统一改用通用附件能力。

2、调整合同费用接口为 study_id 语义，并补充合同、凭证、发票等附件类型的权限映射。

3、将合同费用新建和编辑改为抽屉流程，详情页与列表页复用同一编辑组件。

4、补充数据库迁移、附件列表、合同费用抽屉和权限场景测试，覆盖旧权限移除与新流程。

backend/app/models/contract_fee.py

@@ -34,3 +34,7 @@ class ContractFee(Base):
     updated_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, server_default=func.now(), onupdate=func.now()
     )
+
+    @property
+    def study_id(self) -> uuid.UUID:
+        return self.project_id

backend/app/models/fee_attachment.py

@@ -1,28 +0,0 @@
-from __future__ import annotations
-from typing import Optional
-
-import uuid
-from datetime import datetime
-
-from sqlalchemy import Boolean, DateTime, ForeignKey, Integer, String, Text, func
-from sqlalchemy.dialects.postgresql import UUID
-from sqlalchemy.orm import Mapped, mapped_column
-
-from app.db.base_class import Base
-
-
-class FeeAttachment(Base):
-    __tablename__ = "fee_attachments"
-
-    id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
-    entity_type: Mapped[str] = mapped_column(String(50), nullable=False)
-    entity_id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), nullable=False)
-    file_type: Mapped[str] = mapped_column(String(30), nullable=False)
-    filename: Mapped[str] = mapped_column(String(255), nullable=False)
-    mime_type: Mapped[Optional[str]] = mapped_column(String(100), nullable=True)
-    size: Mapped[int] = mapped_column(Integer, nullable=False)
-    storage_key: Mapped[Optional[str]] = mapped_column(String(500), nullable=True)
-    url: Mapped[Optional[str]] = mapped_column(Text, nullable=True)
-    uploaded_by: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), ForeignKey("users.id"), nullable=False)
-    uploaded_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), nullable=False, server_default=func.now())
-    is_deleted: Mapped[bool] = mapped_column(Boolean, nullable=False, server_default="false")