Fix duplicate session billing and align pricing with DeepSeek ×3.

Serialize billSessionUsage with row locks, expose rates on recharge, add compensation script and admin usage views.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/components/RechargeModal.tsx

@@ -3,6 +3,7 @@ import { createPortal } from 'react-dom';
 import QRCode from 'qrcode';
 import { createRechargeOrder, getBillingConfig, getRechargeOrder } from '../api/client';
 import type { BillingConfig, RechargeOrder } from '../types';
+import { formatBillingRates } from '../utils/billing';
 import { invokeWechatJsapiPay, isWeChatBrowser } from '../utils/wechatPay';
 
 function formatYuan(cents: number) {
@@ -215,6 +216,12 @@ export function RechargeModal({
             <p className="recharge-muted">
               最低充值 {formatYuan(config?.minRechargeCents ?? 500)}，用于 AI 对话按量扣费
             </p>
+            {config && (
+              <p className="recharge-rates">
+                当前费率：{formatBillingRates(config.inputCentsPer1k, config.outputCentsPer1k)}
+                。Agent 模式含工具与记忆，实际消耗通常高于普通聊天。
+              </p>
+            )}
             <div className="recharge-tier-grid">
               {tiers.map((tier) => (
                 <button