feat(P11-P12): 落地治理开关与移动端MVP（G0-G6/H1/M1-M4）

- 新增 Feature Flag 模型、评估服务与管理 API（erp_sync/invoice_verification）\n- 完成 ERP 与验真灰度收口（路由/服务/作业统一判定）\n- 补齐 pay 与手动 sync 的 department_id 灰度上下文\n- 增加管理后台开关页、PWA 基础设施与移动拍照/审批页面\n- 增加回归脚本、安全基线与压测脚本，更新文档与执行合同\n\nIssue: G0,G1,G2,G3,G4,G5,G6,H1,M1,M2,M3,M4

server/src/config.js

@@ -91,6 +91,7 @@ function getConfig() {
     ocrTimeoutMs: Number(process.env.OCR_TIMEOUT_MS ?? 5000),
 
     invoiceVerificationEnabled: process.env.INVOICE_VERIFICATION_ENABLED === "true",
+    invoiceVerificationFlagEnv: process.env.INVOICE_VERIFICATION_ENABLED ?? "false",
     verificationProvider: process.env.VERIFICATION_PROVIDER ?? "mock",
     verificationPollCron: process.env.VERIFICATION_POLL_CRON ?? "* * * * *",
     verificationMaxAttempts: Number(process.env.VERIFICATION_MAX_ATTEMPTS ?? 3),
@@ -102,6 +103,7 @@ function getConfig() {
     budgetControlMode: process.env.BUDGET_CONTROL_MODE ?? "warn",
 
     erpSyncEnabled: process.env.ERP_SYNC_ENABLED === "true",
+    erpSyncFlagEnv: process.env.ERP_SYNC_ENABLED ?? "false",
     erpSyncProvider: process.env.ERP_SYNC_PROVIDER ?? "mock",
     erpSyncEndpoint: process.env.ERP_SYNC_ENDPOINT ?? "",
     erpSyncTimeoutMs: Number(process.env.ERP_SYNC_TIMEOUT_MS ?? 5000),