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/Services/IProvisioningService.cs

@@ -30,13 +30,20 @@ public interface IProvisioningService
     /// <param name="syncUnwatched">Whether unwatched state propagates too.</param>
     /// <param name="syncPlayCount">Whether play counts are raised on watch.</param>
     /// <param name="isDisabled">Whether the group is suspended.</param>
-    /// <returns>The updated group.</returns>
+    /// <param name="inheritParentalRating">Whether the rating cap is the strictest member's.</param>
+    /// <param name="parentalRatingCap">
+    /// The rating cap to use instead, as Jellyfin's numeric score (<c>null</c> for none). Ignored
+    /// when inheriting. Kept within the members' range: see <see cref="SharedGroup.ParentalRatingCap"/>.
+    /// </param>
+    /// <returns>The updated group, with the cap as actually stored.</returns>
     Task<SharedGroup> UpdateGroupAsync(
         Guid sharedUserId,
         IReadOnlyList<Guid> memberIds,
         bool syncUnwatched,
         bool syncPlayCount,
-        bool isDisabled);
+        bool isDisabled,
+        bool inheritParentalRating,
+        int? parentalRatingCap);
 
     /// <summary>
     /// Removes a group, optionally deleting its shared account.