fix: don't delete rows from messages table when the associated member is deleted

PluralKit.Core/Database/Migrations/24.sql

@@ -0,0 +1,10 @@
+-- schema version 24
+-- don't drop message rows when system/member are deleted
+
+alter table messages alter column member drop not null;
+alter table messages drop contstraint messages_member_fkey;
+alter table messages
+    add constraint messages_member_fkey
+    foreign key (member) references members(id) on delete set null;
+
+update info set schema_version = 24;

PluralKit.Core/Database/Repository/ModelRepository.Message.cs

@@ -29,9 +29,13 @@ public partial class ModelRepository
         FullMessage Mapper(PKMessage msg, PKMember member, PKSystem system) =>
             new() { Message = msg, System = system, Member = member };
 
+        var query = "select * from messages"
+            + " left join members on messages.member = members.id"
+            + " left join systems on members.system = systems.id"
+            + " where (mid = @Id or original_mid = @Id)";
+
         var result = await conn.QueryAsync<PKMessage, PKMember, PKSystem, FullMessage>(
-            "select messages.*, members.*, systems.* from messages, members, systems where (mid = @Id or original_mid = @Id) and messages.member = members.id and systems.id = members.system",
-            Mapper, new { Id = id });
+            query, Mapper, new { Id = id });
         return result.FirstOrDefault();
     }