feat(server): 重构报销单服务，优化费用报销流程和数据校验逻辑，包含schema定义和服务实现

server/src/app/schemas/reimbursement.py

@@ -87,6 +87,8 @@ class ExpenseClaimAttachmentRead(BaseModel):
     size_bytes: int
     uploaded_at: datetime | None = None
     previewable: bool = True
+    preview_kind: str = ""
+    preview_url: str = ""
     analysis: ExpenseClaimAttachmentAnalysisRead | None = None
     document_info: ExpenseClaimAttachmentDocumentInfoRead | None = None
     requirement_check: ExpenseClaimAttachmentRequirementRead | None = None