let a link be used by several people instead of dying on first open

The create popup now has a "Let several people use this link" box next to the
expiry controls. Tick it and the link stays redeemable by everyone you send it to
until it expires; leave it and you get the old behaviour, where the first person
to open it is the only one who gets in. The plugin setting that used to be the
only control is now just what the box starts out as, and its label on the config
page says so, because "Default one-use links" explained nothing.

Multi-use did not actually work before this. Two things in Jellyfin stopped it,
and both had to change:

Guests were given MaxActiveSessions = 1, and AuthenticateNewSessionInternal throws
SecurityException once a user is at that limit. The second viewer's redemption
would fail, the record would go to Failed, and cleanup would then delete the guest
account, kicking the first viewer out too. Multi-use links now get 0, which is how
Jellyfin spells "no limit" in that check.

The device id was generated once and reused for every redemption, and
GetAuthorizationToken logs out every existing session for the same user and device
before issuing a token. So even under a raised session cap, each new viewer would
have revoked the previous one's token. Multi-use links now mint a device id per
redemption.

Both viewers of a multi-use link share one temporary account, so they also share
playback position and watched state on the shared title.

README.md

@@ -30,7 +30,10 @@ real user or handing over a login that sees everything.
 
 1. As an admin you open the context menu on a movie, episode, series or season
    and hit **ShareLink**. You choose an expiry (1 hour up to 30 days) and the
-   plugin hands you a link, copied to your clipboard.
+   plugin hands you a link, copied to your clipboard. You also choose there
+   whether the link is single use, which is the default and stops working once
+   the first person opens it, or multi-use, which lets everyone you send it to
+   open it until it expires.
 2. Behind the scenes the plugin tags the shared item with a unique, random tag
    and records the share. Share a series or a season and the tag is applied to
    the whole tree underneath it too - series, seasons and episodes - so the
@@ -125,7 +128,7 @@ All of these live on the plugin's dashboard page:
 | Guest username prefix | Prefix for the throwaway guest accounts (default `share-`) |
 | Allow transcoding / remuxing | Whether guest playback may transcode or remux |
 | Cleanup interval | How often the background cleanup runs |
-| One-use default | Whether new links default to single redemption |
+| Single use by default | How the single-use box starts out in the create popup; it is a per-link choice |
 | Guest lockdown | The web-client confinement described above (on by default) |
 | Guest hidden selectors | CSS selectors hidden from guests, to suppress other plugins' UI |