P7: community flairs, from Lemmy and PieFed

- A post's flairs are read in both dialects: Lemmy 1.0's CommunityPostTag (id, slug, name, description, colour slot)
  and PieFed's lemmy:CommunityTag (display name, text and background colours, whether to blur images). Colours are
  kept only as a colour slot or a hex value.
- A community's own list comes from its `tag` and PieFed's older `lemmy:tagsForPosts`, and fills in a post that names
  only a flair's id and slug, as Lemmy's posts do. Edits and refreshes keep them current.
- Clients get them as `privapub.flairs`.

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

docs/ROADMAP.md

@@ -691,7 +691,9 @@ it, raw where it doesn't.
     ban of a persona refuses its posts there and shows as `blocked_by`), featured; `Warn` and `Resolve` (**done
     2026-10-07**: a warning becomes the persona's `moderation_warning` notification, believed from the community's
     server or its moderators collection, kept as `ForeignAvatar.ModeratorsURL`; a resolution is kept on our report);
-  - votes in and out; link posts; flairs; `Feed` actors; community polls; post `Move`;
+  - votes in and out; link posts; flairs (**done 2026-10-07**: Lemmy 1.0's `CommunityPostTag` and PieFed's
+    `lemmy:CommunityTag`, on posts and communities, described from the community's list, as `privapub.flairs`); `Feed`
+    actors; community polls; post `Move`;
   - the outbound shape Lemmy requires;
   - communities we host announce to the author's own instance too: **done 2026-10-06** (its server's shared inbox,
     whether anyone there follows the community or not: Lemmy keeps its user's post pending until it is announced back);