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>

schema.sql

@@ -354,6 +354,7 @@ CREATE TABLE IF NOT EXISTS h5_usage_records (
   balance_after_cents BIGINT NOT NULL,
   created_at BIGINT NOT NULL,
   KEY idx_h5_usage_user_time (user_id, created_at),
+  KEY idx_h5_usage_created (created_at),
   CONSTRAINT fk_h5_usage_user FOREIGN KEY (user_id) REFERENCES h5_users(id) ON DELETE CASCADE
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;