feat(flags): put /about and /blog behind flags, dark by default

Both features ship dark. Neither is reachable in an environment where
its flag is off, and every flag in this system starts off, so a deploy
of this commit makes both disappear until someone turns them on
deliberately.

Two independent flags rather than one, which makes blog-on-about-off a
reachable state. That state is the whole reason the change is larger
than four notFound() calls: the blog leans on the About page for its
author identity. The Person entity is anchored at /about#tudor, and
that URL 404s while about_page is dark, so a post published in that
state would claim an author resolving to nothing. Worse than having no
named author. Both bylines fall back to unlinked text and the
BlogPosting attributes to the publisher instead, so every combination
of the two flags renders something correct.

Gated: /about, /blog, /blog/[slug], the RSS feed, both footer links,
and the matching content-sitemap entries. A sitemap must never
advertise a URL that 404s. With both dark it emits a valid empty
urlset rather than a 404, because robots.txt names it unconditionally.

Not gated: /admin. Posts have to be writable before the blog is worth
switching on, so flagging the panel would make the flag unflippable.

getFlags takes a revalidate rather than always using the 300s
constant. Reading a flag pins the calling route to the lowest
revalidate among its fetches, and the footer links live in the root
layout, so a naive gate there would have dropped every school and
place page from a weekly cache to a 5-minute one. The layout passes
604800, the floor those routes already declare, and the build confirms
all four SSG route families still prerender. The cost is one-way
latency: pages follow a flip in minutes, footer links within a week.

The e2e journeys follow the existing paired shape from the
admission_distance flag: a lit journey and a dark one for each flag,
reading state from whether /about and /blog respond rather than from
/api/flags, which another journey asserts is not publicly reachable.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DXnXQKnPpZBBP61fBQiFkq

nextjs-app/docs/PUBLISHING.md

@@ -4,6 +4,26 @@ The blog is Payload CMS, running inside the Next.js app. There is no separate
 service and no second deploy. Writing a post is done in the browser and takes
 effect on the live site within seconds.
 
+## Before any of this: the flags
+
+`/blog` and `/about` are behind feature flags (`blog` and `about_page`), and
+every flag in this system starts off. While `blog` is dark, `/blog`, every post
+page and the RSS feed return 404, and neither appears in the sitemap or the
+footer. Posts still save normally, because `/admin` is deliberately **not**
+flagged: you have to be able to write a post before there is anything worth
+switching on.
+
+So a new post published to a dark blog is invisible, and that is working as
+intended, not a bug. Flip the flag in Unleash when the content is ready.
+Staging and production hold their own values (`development` and `production`
+environments), so you can light it on staging first. The page follows a flip
+within five minutes; the footer link takes up to a week, because it renders in
+the root layout and is cached at the same weekly floor as the school corpus.
+
+Both flags are temporary scaffolding, like every flag here: a test starts
+failing once one is older than 90 days, at which point either the feature is
+permanent and the flag comes out, or it was never going to ship.
+
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