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/utils/billing.ts

@@ -0,0 +1,17 @@
+/** Format CNY cents-per-1k-tokens as readable yuan string. */
+export function formatYuanPer1kTokens(centsPer1k: number) {
+  const yuan = centsPer1k / 100;
+  if (yuan >= 0.01) return `¥${yuan.toFixed(2)}/1k`;
+  return `¥${yuan.toFixed(3)}/1k`;
+}
+
+/** Approximate yuan per million tokens from cents-per-1k rate. */
+export function formatYuanPerMillionTokens(centsPer1k: number) {
+  const yuanPerM = (centsPer1k / 100) * 1000;
+  if (yuanPerM >= 1) return `约 ¥${yuanPerM.toFixed(1).replace(/\.0$/, '')}/百万`;
+  return `约 ¥${yuanPerM.toFixed(2)}/百万`;
+}
+
+export function formatBillingRates(inputCentsPer1k: number, outputCentsPer1k: number) {
+  return `输入 ${formatYuanPer1kTokens(inputCentsPer1k)}（${formatYuanPerMillionTokens(inputCentsPer1k)}），输出 ${formatYuanPer1kTokens(outputCentsPer1k)}（${formatYuanPerMillionTokens(outputCentsPer1k)}）`;
+}