feat(learning): harden access and practice sessions

Tiku.Application/StudyContent/StudyContentQueryModels.cs

@@ -13,7 +13,8 @@ public sealed record StudyContentFilter(
     Guid? ContentNodeId = null,
     string? Keyword = null,
     bool IncludeContent = false,
-    int? Limit = null);
+    int? Limit = null,
+    IReadOnlyCollection<Guid>? AllowedRegionIds = null);
 
 public sealed record VocabularyUnitCatalogItem(
     Guid Id,
@@ -81,4 +82,4 @@ public sealed record HandbookEntryCatalogItem(
     string? Content,
     JsonElement Tags,
     int Order,
-    JsonElement Metadata);
+    JsonElement Metadata);