XP series of one account

Built for polling every 2–10 s (the live map, Home Assistant). Limited to 1 request per 1 s per key, on top of the general limit. Each account carries `id`, `name`, `type`, `type_label`, `owner` and `categories` (and `account_hash` for service keys); every other field belongs to one category and is **omitted** when the key can’t read that category on the account, and **null** when it can but the plugin never sent it. Send the last `ETag` as `If-None-Match`: while nothing you would see has changed, the answer is `304 Not Modified` without a body (it still counts towards the rate limits). With `since` (the previous `meta.last_modified`), only accounts that changed after it are returned, re-sending those that changed up to 30 s before it; accounts whose `activity` the key can’t read are always returned, and an account that leaves the key’s scope simply stops appearing, so fetch without `since` now and then. A location older than 2 minutes has `stale: true`.