feat: 移除“租户管理员为用户充值 / 每租户一套余额”能力：余额统一为全局用户余额

backend/database/migrations/20251215084449_users.sql

@@ -10,9 +10,17 @@ CREATE TABLE IF NOT EXISTS users(
     roles text[] NOT NULL DEFAULT ARRAY['user'],
     status varchar(50) NOT NULL DEFAULT 'active',
     metas jsonb NOT NULL DEFAULT '{}',
+    balance bigint NOT NULL DEFAULT 0,
+    balance_frozen bigint NOT NULL DEFAULT 0,
     verified_at timestamptz
 );
 
+COMMENT ON COLUMN users.balance IS '全局可用余额：分/最小货币单位；用户在所有已加入租户内共享该余额；默认 0';
+COMMENT ON COLUMN users.balance_frozen IS '全局冻结余额：分/最小货币单位；用于下单冻结等；默认 0';
+
+CREATE INDEX IF NOT EXISTS ix_users_balance ON users(balance);
+CREATE INDEX IF NOT EXISTS ix_users_balance_frozen ON users(balance_frozen);
+
 -- +goose StatementEnd
 -- +goose Down
 -- +goose StatementBegin