Persona and group ids say which day, nothing more

An ObjectId carries its creation second and a per-process counter, so two
avatars made one after the other by the same login got ids a few counts
apart: a link between personas that every Mastodon client would have
seen. Avatar and Group now generate ids from the UTC day plus eight random
bytes (still valid ObjectIds), and a remote post's id is made from its
published time with a random tail, so posts page in the order they were
written rather than the order they arrived.

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

PrivaPub/Infrastructure/Ids/PrivacyIds.cs

@@ -0,0 +1,20 @@
+using System.Buffers.Binary;
+using System.Security.Cryptography;
+
+namespace PrivaPub.Infrastructure.Ids
+{
+	public static class PrivacyIds
+	{
+		public static string ForDay(DateTime when) => At(DateTime.SpecifyKind(when, DateTimeKind.Utc).Date);
+
+		public static string At(DateTime when)
+		{
+			var seconds = Math.Clamp(new DateTimeOffset(DateTime.SpecifyKind(when, DateTimeKind.Utc)).ToUnixTimeSeconds(), 0,
+				DateTimeOffset.UtcNow.AddDays(1).ToUnixTimeSeconds());
+			Span<byte> bytes = stackalloc byte[12];
+			BinaryPrimitives.WriteInt32BigEndian(bytes, (int)seconds);
+			RandomNumberGenerator.Fill(bytes[4..]);
+			return Convert.ToHexStringLower(bytes);
+		}
+	}
+}