feat(billing): make metering formula admin-configurable

Allow margin/FX/cost-mode knobs to be overridden via h5_billing_admin_config so memind_adm can adjust DeepSeek billing without editing env.

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

server.mjs

@@ -412,6 +412,8 @@ async function bootstrapUserAuth() {
       });
     subscriptionService =
       authServices.subscriptionService;
+    const billingConfigService =
+      authServices.billingConfigService;
     userAuth = authServices.userAuth;
     sessionAccess = authServices.sessionAccess;
     wechatPayClient = authServices.wechatPayClient;
@@ -501,6 +503,7 @@ async function bootstrapUserAuth() {
         sessionStreamStore,
         llmProviderService,
         subscriptionService,
+        billingConfigService,
         sessionSnapshotService,
         conversationMemoryService,
         memoryV2,