Usage counted in your own month, invites in your own time zone, and a store that counts stock

A round of fixes to the parts of Solstral that count things.

• Usage counts in your month. On a paid plan your monthly allowances now run with your invoice (the 15th to the 14th, say); on Free they run with the calendar month. Either way the month starts at midnight where your workspace is, not midnight in London, and the Usage tab says which dates it is counting.
• Meeting invites show the host's time. Calendar invites now carry the host's time zone, so Outlook, Google and Apple show the meeting at the host's local time and keep it right across a daylight-saving change. All-day events stay all-day.
• Your own email signing key is renewed for you. If your domain signs with its own key, we now prepare a new one each year, tell you the record to add, switch only once it is published, and tell you when the old one can go.
• The store counts stock. Set how many of a product you have and the last one can only be sold once; at zero the product shows Sold out.
• API limits by plan. How many requests a minute an API key may make, and how many it may have running at once, now follow your plan (60 to 1,200 a minute).