Inherit parental restrictions on shared accounts, with a chosen rating cap

A shared account previously inherited its members' library access but
none of their content restrictions, so a child could log into "alice+kid"
with their own password and get around their own rating cap.

The shared account now gets the strictest member's parental rating,
unrated-item block, blocked tags and allowed tags, recomputed at
creation, on membership change and at startup. An admin can raise the
rating cap on a slider between the strictest and the loosest member;
unrated and tag rules stay strictest-wins.

What makes raising the cap safe is the unlock rule: after a member's
password matches, both users' live policies are compared and the login
is refused if the account is looser than the member on any field. So
raising the cap above the child's rating means the child's password no
longer opens the account, while the parent's still does. The same rule
bounds the slider - past the loosest member nobody could unlock the
account - so a chosen cap is clamped back into range whenever applied.

Allowed tags need care: Jellyfin reads an empty list as "no whitelist",
so an empty intersection of members' whitelists is written as a sentinel
tag no item carries. Access schedules and channels are not inherited yet.

The shared account is never an administrator. Groups created at the
login screen always inherit and are restricted before the first session
exists. The dashboard shows each member's cap, who a chosen cap shuts
out, and the restrictions in effect, and gains a per-group edit form for
the sync options.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Jellyfin.Plugin.WatchedTogether/Configuration/SharedGroup.cs

@@ -41,6 +41,30 @@ public class SharedGroup
     /// </summary>
     public bool SyncPlayCount { get; set; }
 
+    /// <summary>
+    /// Gets or sets a value indicating whether the shared account's parental rating cap is the
+    /// strictest member's. When false, <see cref="ParentalRatingCap"/> is used instead.
+    /// </summary>
+    /// <remarks>
+    /// Groups created at the login screen always inherit; choosing a cap is a dashboard-only
+    /// action so nobody can widen access from the login screen. Unrated-item blocks and tag rules
+    /// are always the strictest member's - they have no meaningful "level" to choose.
+    /// </remarks>
+    public bool InheritParentalRating { get; set; } = true;
+
+    /// <summary>
+    /// Gets or sets the parental rating cap, as Jellyfin's numeric score, to use when
+    /// <see cref="InheritParentalRating"/> is false. <c>null</c> means no cap.
+    /// </summary>
+    /// <remarks>
+    /// Kept within the range spanned by the members' own caps whenever it is applied: no lower than
+    /// the strictest member (that would just be inheriting) and no looser than the loosest member
+    /// (nobody could unlock the account past that, since a member may only unlock an account at
+    /// least as restricted as they are). Wherever it sits in that range, members stricter than it
+    /// can no longer unlock the account - that is the whole point of choosing one.
+    /// </remarks>
+    public int? ParentalRatingCap { get; set; }
+
     /// <summary>
     /// Gets or sets a value indicating whether this group is suspended. A group drops out of both
     /// authentication and sync while disabled - set automatically if it falls below two members.