Settle on a five-minute public cache and drop the purge rationale

The edge-purge integration was reverted in the previous commit; this
finishes the job. Public pages, the feed and /issues.json now say
`public, max-age=300` for browsers and shared caches alike, the code
comments and README explain why the longer s-maxage plus purge design
was dropped, and the rollout runbook loses its purge step, marks the
HTML cache rule optional, and warns to keep the record DNS-only until
the Universal SSL certificate is active.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Va5eMEmWEnjMXBsBob5FDW

src/server.rs

@@ -304,7 +304,7 @@ async fn handle_issues_json(State(state): State<AppState>) -> Response {
         })
         .collect();
     match serde_json::to_string_pretty(&issues) {
-        // Only publishing changes this, and publishing purges the edge (§3.12).
+        // Only publishing changes this; five minutes of staleness is fine (§3.12).
         Ok(body) => (
             StatusCode::OK,
             [