One Post model for every visibility, read from notes the way peers write them

Post gains what federation and the Mastodon API need: Visibility (Public,
Unlisted, FollowersOnly, Direct, Circle, LocalGeo), the author's account
id, to/cc, the Create's id, url, context, quote, InReplyToURI and the
parent's author, a separate SpoilerText next to the title, language,
mentions, hashtags, remote attachments (alt text, blurhash, focus, size),
reply/favourite/reblog counters, revisions, EditedAt and DeletedAt.

Direct messages are Posts with Visibility Direct and a ConversationId;
migration _005 copies DmPost rows across with their ids and fills the new
fields of existing posts. DmPost is left in place so a rollback still sees
the old messages.

Inbound:
- NoteParser reads Note, Article, Page, Question and media types: content,
  then contentMap, then _misskey_content; summary as the spoiler and name
  as the title; Mention and Hashtag tags; attachments; a PeerTube-style
  list attribution prefers the person over the channel; quote URIs.
- Addressing classifies like Mastodon, finding followers-only by the
  author's own followers URL (now stored on ForeignAvatar), not by a
  "/followers" suffix.
- Create keeps a post when a local persona is addressed or mentioned, when
  it replies to a local post (the parent's reply count goes up) or when a
  community it follows is addressed; an unsolicited public post is not
  stored. Update keeps the previous version as a revision.

The outbox and object endpoints serve only Public and Unlisted posts.

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

PrivaPub/Federation/Inbox/Handlers/FollowHandler.cs

@@ -15,7 +15,6 @@ using System.Text.Json.Nodes;
 using static PrivaPub.Federation.Inbox.ForeignMembers;
 using static PrivaPub.Federation.Objects.ActivityJson;
 
-using DmPostEntity = PrivaPub.Models.Post.DmPost;
 using PostEntity = PrivaPub.Models.Post.Post;
 
 namespace PrivaPub.Federation.Inbox.Handlers