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

docs/ROADMAP.md

@@ -408,12 +408,15 @@ details view of where each object came from. So the server keeps everything it r
 it, raw where it doesn't.
 
 #### P5 Lose nothing (wire tolerance and full objects)
-- **Cheap fixes that are wrong today:**
-  - `summary` is a content warning only on a `Note`, or when `sensitive` is set; elsewhere it is an excerpt or
-    description (INTEROP W2).
-  - Persona and group usernames must match Mastodon's and Misskey's pattern.
-  - Define every term we emit in our JSON-LD context.
-  - `Vary: Accept`; every activity id dereferences.
+- **Cheap fixes that were wrong** (done in v1.7.0):
+  - `summary` is a content warning only on a `Note`/`Question`, or when `sensitive` is set; elsewhere it is an excerpt
+    (INTEROP W2). Remote titles now show in the Mastodon API for non-Note objects.
+  - Persona usernames match Mastodon's and Misskey's pattern (groups already did).
+  - Every term we emit is defined in our JSON-LD context.
+  - `Vary: Accept`.
+  - Still open: every activity id dereferences.
+- **Owner decisions that were small** (done in v1.7.0): blocks federate (`Block`, `Undo{Block}`); a persona's and a
+  group's `published` is a random day up to two weeks before its creation (migration `_007`).
 - **Parsing every shape:**
   - `url`, `icon`, `image`, `attachment` and `attributedTo` as a value, an object or an array (W1).
   - A Markdown `content` and `source` (W3).