feat: 预算版本管理API — 修复前端'加载版本失败'(budgetApi.versions未定义+后端无端点)

- 后端: GET /budget/versions(版本列表) POST /versions/submit /versions/approve /versions/diff
- 前端: budgetApi.versions/versionSubmit/versionApprove/versionDiff
- 数据: budget_plans已有v2.0 active 24条
- 验证: 版本列表200(v2.0 24KPI 预算730万) 版本对比200

frontend/src/api/index.ts

@@ -178,6 +178,11 @@ export const budgetApi = {
   autoDecompose: (data: any) => api.post('/budget/auto-decompose', data),
   deviationReport: (params?: any) => api.get('/budget/deviation-report', { params }),
   methodComparison: (data: any) => api.post('/budget/method-comparison', data),
+  // 版本管理
+  versions: (params?: any) => api.get('/budget/versions', { params }),
+  versionSubmit: (data: any) => api.post('/budget/versions/submit', data),
+  versionApprove: (data: any) => api.post('/budget/versions/approve', data),
+  versionDiff: (data: any) => api.post('/budget/versions/diff', data),
   // 持续规划
   getConfig: () => api.get('/budget/config'),
   setConfig: (data: any) => api.post('/budget/config', data),