Announced pages: event windows, header announcements, summary tasks

A question may carry event: {starts, duration, place}. While the
window is open the page is announced in a strip at the top of every
header (name, when, 'in 3 days'), soonest first, and kept out of the
footer nav; when it closes the page becomes a followup - only a
visitor carrying an answer chain still sees it.

announce.rs keeps one record per event page in the runtime-owned
portal_events bucket (built-in state graph: announced ->
awaiting_summary -> summarized, content may override) and, on a
one-minute idempotent sweep, moves ended windows to awaiting_summary,
publishing the transition on portal.answers.submitted as 'Summary
due' - the post-what-happened task a review desk picks up. Lint
warns when event pages exist but nothing reads portal_events.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/lib.rs

@@ -9,6 +9,9 @@ pub mod resource;
 #[cfg(feature = "ssr")]
 pub mod aggregates;
 
+#[cfg(feature = "ssr")]
+pub mod announce;
+
 #[cfg(feature = "ssr")]
 pub mod server;