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/web/mod.rs

@@ -771,7 +771,7 @@ mod tests {
         assert!(anonymous.headers().get(header::SET_COOKIE).is_none());
         assert_eq!(
             anonymous.headers().get(header::CACHE_CONTROL).unwrap(),
-            "public, max-age=300, s-maxage=86400"
+            "public, max-age=300"
         );
         assert_eq!(
             anonymous.headers().get("speculation-rules").unwrap(),