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/DynamicGroupService.cs

@@ -31,6 +31,7 @@ public class DynamicGroupService : IDynamicGroupService
 {
     private readonly IUserManager _userManager;
     private readonly IProvisioningService _provisioningService;
+    private readonly IRestrictionService _restrictionService;
     private readonly ICryptoProvider _cryptoProvider;
     private readonly ILogger<DynamicGroupService> _logger;
 
@@ -39,16 +40,19 @@ public class DynamicGroupService : IDynamicGroupService
     /// </summary>
     /// <param name="userManager">The user manager.</param>
     /// <param name="provisioningService">The provisioning service.</param>
+    /// <param name="restrictionService">The content restriction service.</param>
     /// <param name="cryptoProvider">The crypto provider.</param>
     /// <param name="logger">The logger.</param>
     public DynamicGroupService(
         IUserManager userManager,
         IProvisioningService provisioningService,
+        IRestrictionService restrictionService,
         ICryptoProvider cryptoProvider,
         ILogger<DynamicGroupService> logger)
     {
         _userManager = userManager;
         _provisioningService = provisioningService;
+        _restrictionService = restrictionService;
         _cryptoProvider = cryptoProvider;
         _logger = logger;
     }
@@ -155,6 +159,17 @@ public class DynamicGroupService : IDynamicGroupService
                 return null;
             }
 
+            // The same unlock rule the authentication provider applies to a resolved account: an
+            // existing group may have had its rating cap raised in the dashboard since it was created.
+            if (!_restrictionService.IsAtLeastAsStrict(existingUser, matched))
+            {
+                _logger.LogInformation(
+                    "Rejected login for {Username} by member {MemberUsername}: the shared account is less restricted than the member",
+                    existingUser.Username,
+                    matched.Username);
+                return null;
+            }
+
             _logger.LogInformation(
                 "Login as {Entered} resolved to the existing shared account {Username}",
                 enteredUsername,
@@ -165,8 +180,9 @@ public class DynamicGroupService : IDynamicGroupService
 
         // Passing no name lets provisioning generate the canonical alphabetically-sorted one, so
         // the account is named the same whichever order the members were typed in.
-        // The account is limited to the libraries all named members share, so creating one at the
-        // login screen cannot grant anybody access they did not already have.
+        // The account is limited to the libraries all named members share and inherits their
+        // strictest restrictions before this returns, so creating one at the login screen cannot
+        // grant anybody access they did not already have - not even for the session it creates.
         var group = await _provisioningService.CreateGroupAsync(memberIds, null).ConfigureAwait(false);
 
         var sharedUser = _userManager.GetUserById(group.SharedUserId);