Manually trigger the daily traffic digest email

Ad-hoc trigger for the daily digest. Useful for verifying the email format before the next scheduled tick fires.

date — optional ISO YYYY-MM-DD. Reports on the named local calendar day (DIGEST_TZ). Defaults to yesterday. to — optional override recipient. When set, ONLY that address receives the mail (useful for "send me a preview"); the normal scheduled run goes to every admin.