P5, first batch: summaries stop hiding articles, personas get reachable names, blocks federate

- A remote `summary` is a content warning only on a Note or Question, or when `sensitive` is set. On Articles,
  Events, Videos, Pages and Audio it is an excerpt (WordPress teasers, Mobilizon dates and places, Mbin titles) and
  is now kept as `Post.Excerpt` instead of hiding the post. `Post.ObjectType` records the remote type, and the
  Mastodon API shows a remote non-Note object's title above its body again.
- Persona usernames must match `^[a-z0-9_]+$`, as groups already did; a name outside it was unreachable from
  Mastodon and Misskey.
- `postingRestrictedToMods` is defined in our JSON-LD context (Iceshrimp.NET drops undefined terms).
- `Vary: Accept` on actor and object URLs.
- Owner decision: blocks federate. A block sends `Block`, an unblock `Undo{Block}`; checked live against GoToSocial.
- Owner decision: a persona's and a group's `published`, and the day in new ids, is a random day up to two weeks
  before creation, so personas made the same day no longer share a date. Migration _007 gives existing ones theirs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

PrivaPub.ClientModels/Constants.cs

@@ -9,5 +9,6 @@
 		public const int GroupInvitationLength = 64;
 
 		public const string PasswordRegex = @"^(?=.*)(?=.*[a-z])(?=.*[A-Z])(?!.*\s).*$";
+		public const string UserNameRegex = "^[a-z0-9_]+$";
 	}
 }

PrivaPub.ClientModels/Group/InsertGroupForm.cs

@@ -12,7 +12,7 @@ namespace PrivaPub.ClientModels.Group
 
 		[Required(ErrorMessageResourceName = "Required", ErrorMessageResourceType = typeof(ErrorsResource)),
 			StringLength(32, MinimumLength = 3, ErrorMessageResourceName = "StringLengthMinMax", ErrorMessageResourceType = typeof(ErrorsResource)),
-			RegularExpression("^[a-z0-9_]+$", ErrorMessageResourceName = "EmptySpacesNotAllowed", ErrorMessageResourceType = typeof(ErrorsResource)),
+			RegularExpression(Constants.UserNameRegex, ErrorMessageResourceName = "InvalidUserName", ErrorMessageResourceType = typeof(ErrorsResource)),
 			NoWhiteSpaces]
 		public string UserName { get; set; }
 

PrivaPub.ClientModels/InvitationLoginForm.cs

@@ -12,7 +12,7 @@ namespace PrivaPub.ClientModels
 		public string InvitationCode { get; set; }
 
 		[StringLength(32, MinimumLength = 3, ErrorMessageResourceName = "StringLengthMinMax", ErrorMessageResourceType = typeof(ErrorsResource)),
-			RegularExpression("^[a-z0-9_]+$", ErrorMessageResourceName = "EmptySpacesNotAllowed", ErrorMessageResourceType = typeof(ErrorsResource))]
+			RegularExpression(Constants.UserNameRegex, ErrorMessageResourceName = "InvalidUserName", ErrorMessageResourceType = typeof(ErrorsResource))]
 		public string AvatarUserName { get; set; }
 
 		[StringLength(64, ErrorMessageResourceName = "MaxLengthString", ErrorMessageResourceType = typeof(ErrorsResource))]

PrivaPub.ClientModels/Resources/ErrorsResource.Designer.cs

@@ -105,6 +105,15 @@ namespace PrivaPub.ClientModels.Resources {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to Use only lower-case letters, digits and underscores..
+        /// </summary>
+        public static string InvalidUserName {
+            get {
+                return ResourceManager.GetString("InvalidUserName", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to Invalid password, upper-case, lower-case and number characters required..
         /// </summary>

PrivaPub.ClientModels/Resources/ErrorsResource.it.resx

@@ -132,6 +132,9 @@
   <data name="InvalidEmail" xml:space="preserve">
     <value>{0} è invalida.</value>
   </data>
+  <data name="InvalidUserName" xml:space="preserve">
+    <value>Usa solo lettere minuscole, cifre e trattini bassi.</value>
+  </data>
   <data name="InvalidPassword" xml:space="preserve">
     <value>Password non valida, deve avere almeno un carattere grande, un carattere piccolo ed un numero.</value>
   </data>

PrivaPub.ClientModels/Resources/ErrorsResource.resx

@@ -132,6 +132,9 @@
   <data name="InvalidEmail" xml:space="preserve">
     <value>{0} is invalid.</value>
   </data>
+  <data name="InvalidUserName" xml:space="preserve">
+    <value>Use only lower-case letters, digits and underscores.</value>
+  </data>
   <data name="InvalidPassword" xml:space="preserve">
     <value>Invalid password, upper-case, lower-case and number characters required.</value>
   </data>

PrivaPub.ClientModels/User/Avatar/InsertAvatarForm.cs

@@ -20,6 +20,7 @@ namespace PrivaPub.ClientModels.User.Avatar
 
 		[Required(ErrorMessageResourceName = "Required", ErrorMessageResourceType = typeof(ErrorsResource)),
 			StringLength(Constants.MaxAvatarNameLength, ErrorMessageResourceName = "StringLength", ErrorMessageResourceType = typeof(ErrorsResource)),
+			RegularExpression(Constants.UserNameRegex, ErrorMessageResourceName = "InvalidUserName", ErrorMessageResourceType = typeof(ErrorsResource)),
 				Display(Name = nameof(UserName), ResourceType = typeof(FieldsNameResource))]
 		public string UserName { get; set; }//preferredUsername