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>

db.mjs

@@ -255,6 +255,12 @@ export async function migrateSchema(pool) {
     `ALTER TABLE h5_payment_orders
      MODIFY pay_mode ENUM('native', 'h5', 'jsapi') NOT NULL DEFAULT 'native'`,
   );
+
+  if (!(await indexExists(pool, 'h5_usage_records', 'idx_h5_usage_created'))) {
+    await pool.query(
+      `ALTER TABLE h5_usage_records ADD KEY idx_h5_usage_created (created_at)`,
+    );
+  }
 }
 
 export async function initSchema(pool) {