cap how many people can watch one multi-use link at once

New setting, ten by default, zero for no limit. Single-use links are unaffected,
they are one viewer by definition.

The catch is what happens at the ceiling. Jellyfin throws SecurityException once a
user is at MaxActiveSessions, and that was landing in the generic handler, which
marks the record failed and runs cleanup, which deletes the guest account. So
without care, adding a ceiling would mean the eleventh person to open a link kicks
out the ten already watching and destroys the link. Capacity is caught separately
now: the record goes back to the state it was in, nothing is torn down, and the
new arrival gets a 503 page inviting them to try again.

Worth being honest that this caps how many people can start watching at once, not
how many ever get in: each redemption issues its own session token that keeps
working until the link is revoked or expires. Revoke is still the hard stop.

README picks up the multi-use option, the new setting, and a section on what a
multi-use link does and does not protect, plus the known limits around the token
in the query string, the unthrottled redeem endpoint, and the tag being hidden in
the web UI only.

Jellyfin.Plugin.ShareLinks/Services/JellyfinGuestUserService.cs

@@ -187,9 +187,9 @@ public sealed class JellyfinGuestUserService
             EnabledFolders = Array.Empty<Guid>(),
             EnablePublicSharing = false,
             LoginAttemptsBeforeLockout = -1,
-            // One viewer for a one-use link. A multi-use link needs a session per
-            // viewer, and 0 is how Jellyfin spells "no limit" in its session check.
-            MaxActiveSessions = record.OneUse ? 1 : 0,
+            // One viewer for a one-use link. A multi-use link gets the configured
+            // ceiling, where 0 is how Jellyfin spells "no limit" in its session check.
+            MaxActiveSessions = record.OneUse ? 1 : Math.Max(config.MaxConcurrentViewers, 0),
             BlockUnratedItems = Array.Empty<UnratedItem>()
         };