feat: 新增预算后端服务与差旅风险规则库

后端新增预算模型、端点和服务模块，支持预算 CRUD 和余额
查询，清理旧生成规则文件并替换为按严重等级分类的差旅风
险规则库，优化认证权限和报销单访问策略，新增财务规则目
录和演示数据构建脚本，前端预算中心增加对话框交互，完善
审计页面运行时模型和元数据展示，补充单元测试。

web/src/services/budgets.js

@@ -0,0 +1,30 @@
+import { apiRequest } from './api.js'
+
+function buildQuery(params = {}) {
+  const search = new URLSearchParams()
+  Object.entries(params || {}).forEach(([key, value]) => {
+    if (typeof value === 'undefined' || value === null || value === '') return
+    search.set(key, String(value))
+  })
+  const query = search.toString()
+  return query ? `?${query}` : ''
+}
+
+export function fetchBudgetSummary(params = {}) {
+  return apiRequest(`/budgets/summary${buildQuery(params)}`)
+}
+
+export function fetchBudgetAllocations(params = {}) {
+  return apiRequest(`/budgets/allocations${buildQuery(params)}`)
+}
+
+export function createBudgetAllocation(payload = {}) {
+  return apiRequest('/budgets/allocations', {
+    method: 'POST',
+    body: JSON.stringify(payload)
+  })
+}
+
+export function fetchBudgetTransactions(allocationId) {
+  return apiRequest(`/budgets/allocations/${encodeURIComponent(String(allocationId || '').trim())}/transactions`)
+}