feat: 增加差旅报销标准测算和财务终审流程

新增差旅报销测算接口及 Spreadsheet 规则解析，审批流程拆分
直属领导审批与财务终审两阶段并细分权限，修复 PDF 文本层
缺失时自动回退 OCR，提交后清理关联会话，前端适配审批流
交互并补充单元测试。

server/src/app/schemas/reimbursement.py

@@ -41,6 +41,7 @@ class ExpenseClaimItemRead(BaseModel):
     item_location: str
     item_amount: Decimal
     invoice_id: str | None
+    is_system_generated: bool = False
     created_at: datetime
     updated_at: datetime
 
@@ -157,11 +158,41 @@ class ExpenseClaimApprovalPayload(BaseModel):
     opinion: str | None = Field(default=None, max_length=500)
 
 
+class TravelReimbursementCalculatorRequest(BaseModel):
+    days: int = Field(ge=1, le=365)
+    location: str = Field(min_length=1, max_length=120)
+    grade: str | None = Field(default=None, max_length=30)
+
+
+class TravelReimbursementCalculatorResponse(BaseModel):
+    days: int
+    location: str
+    matched_city: str
+    city_tier: str
+    grade: str
+    grade_band: str
+    grade_band_label: str
+    hotel_rate: Decimal
+    hotel_amount: Decimal
+    allowance_region: str
+    meal_allowance_rate: Decimal
+    basic_allowance_rate: Decimal
+    total_allowance_rate: Decimal
+    allowance_amount: Decimal
+    total_amount: Decimal
+    rule_name: str
+    rule_version: str
+    formula_text: str
+    summary_text: str
+
+
 class ExpenseClaimAttachmentActionResponse(BaseModel):
     message: str
     claim_id: str
     item_id: str
     invoice_id: str | None = None
+    item_amount: Decimal | None = None
+    claim_amount: Decimal | None = None
     attachment: ExpenseClaimAttachmentRead | None = None