Ticket to download one data.

The reason why this table has site field instead of shop is this downloading cgi would be extracted from cassiopeia.

Column	Type	Constraint	Description
site	TEXT	PRIMARY KEY	Site ID. (see shop.username)
id	INTEGER		Identifier.
data_id	INTEGER	NOT NULL	Download data ID. (see downloadable_data.id)
code	TEXT	NOT NULL, UNIQUE	Identifier to certify download request. Randomized string.
remaining	INTEGER		Remaining number of download. NULL means unlimited.
expires	TIMESTAMP		When this ticket will be expired. NULL means no expiry.

CREATE TABLE download_ticket(
site        TEXT,
id          INTEGER,
data_id     INTEGER  NOT NULL,
code        TEXT     NOT NULL  UNIQUE,
remaining   INTEGER,
expires     TIMESTAMP,
PRIMARY KEY(site, id)
);