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>

billing.test.mjs

@@ -1,6 +1,10 @@
 import test from 'node:test';
 import assert from 'node:assert/strict';
-import { computeDeltaCostCents, normalizeTokenState } from './billing.mjs';
+import {
+  computeDeltaCostCents,
+  loadBillingConfig,
+  normalizeTokenState,
+} from './billing.mjs';
 
 const config = {
   useBackendCost: false,
@@ -10,6 +14,43 @@ const config = {
   minBillCents: 1,
 };
 
+test('loadBillingConfig defaults to DeepSeek Flash × 3 at USD/CNY 7.2', () => {
+  const savedInput = process.env.H5_BILL_INPUT_CENTS_PER_1K;
+  const savedOutput = process.env.H5_BILL_OUTPUT_CENTS_PER_1K;
+  const savedRate = process.env.H5_USD_CNY_RATE;
+  delete process.env.H5_BILL_INPUT_CENTS_PER_1K;
+  delete process.env.H5_BILL_OUTPUT_CENTS_PER_1K;
+  process.env.H5_USD_CNY_RATE = '7.2';
+  try {
+    const billing = loadBillingConfig();
+    assert.ok(Math.abs(billing.inputCentsPer1k - 0.3024) < 1e-9);
+    assert.ok(Math.abs(billing.outputCentsPer1k - 0.6048) < 1e-9);
+  } finally {
+    if (savedInput == null) delete process.env.H5_BILL_INPUT_CENTS_PER_1K;
+    else process.env.H5_BILL_INPUT_CENTS_PER_1K = savedInput;
+    if (savedOutput == null) delete process.env.H5_BILL_OUTPUT_CENTS_PER_1K;
+    else process.env.H5_BILL_OUTPUT_CENTS_PER_1K = savedOutput;
+    if (savedRate == null) delete process.env.H5_USD_CNY_RATE;
+    else process.env.H5_USD_CNY_RATE = savedRate;
+  }
+});
+
+test('computeDeltaCostCents bills DeepSeek Flash × 3 defaults with min charge', () => {
+  const billing = {
+    useBackendCost: false,
+    usdCnyRate: 7.2,
+    inputCentsPer1k: 0.3024,
+    outputCentsPer1k: 0.6048,
+    minBillCents: 1,
+  };
+  const previous = { lastInputTokens: 0, lastOutputTokens: 0 };
+  const current = normalizeTokenState({
+    accumulatedInputTokens: 1000,
+    accumulatedOutputTokens: 500,
+  });
+  assert.equal(computeDeltaCostCents(previous, current, billing), 1);
+});
+
 test('normalizeTokenState reads camelCase fields', () => {
   const state = normalizeTokenState({
     inputTokens: 10,