fix(cache): revalidate pages/media instead of 7-day browser cache

system.pages.expires was 604800 (Grav default), so browsers cached feed HTML and
entry media for a week with no revalidation — deleted entries lingered as ghost
cards (images 404), and edited/new photos didn't appear until a hard refresh. Set
expires: 0 → Cache-Control: no-cache, must-revalidate. With etag already on, an
unchanged page returns a cheap 304; changed content shows immediately. Also fixes
reorder staleness (photo-N.jpg is reused across reorders at the same URL). The
server-side page cache (cache.enabled) is untouched.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

config/system.yaml

@@ -76,7 +76,13 @@ pages:
     - rss
     - atom
   append_url_extension: null
-  expires: 604800
+  # expires: 0 → Cache-Control: max-age=0, so the browser (and any CDN) revalidates
+  # each load instead of serving up to 7 days stale. With etag on, an unchanged
+  # page returns a cheap 304; an edited/deleted/new entry shows immediately. This
+  # matters because the owner edits the live site and entry media reuses filenames
+  # (photo-N.jpg) across reorders. Server-side page cache (cache.enabled) is
+  # unaffected and still does the heavy lifting.
+  expires: 0
   cache_control: null
   last_modified: false
   etag: true