Add element-level opt-out for page caching

[robinboening]

Closes #3973.

This adds a page_cache option to element definitions, allowing individual elements to opt a page out of HTTP page caching:

- name: restricted_downloads
  page_cache: false

When a page contains an element definition with page_cache: false, Alchemy now always renders the page instead of relying on the page-cache. This lets dynamic or permission-sensitive elements execute and apply their own fragment cache keys.

The opt-out uses Cache-Control: no-store rather than no-cache, because no-cache may still allow stored responses to be revalidated and return 304, which would skip rendering the element entirely.

The option defaults to true and is kept as definition-only metadata, so existing element definitions and page cache behavior remain unchanged unless an element explicitly opts out.

Checklist

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have added tests to cover this change

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.27%. Comparing base (4ccf434) to head (555daa2).
⚠️ Report is 40 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3980      +/-   ##
==========================================
+ Coverage   98.26%   98.27%   +0.01%     
==========================================
  Files         350      351       +1     
  Lines        9184     9241      +57     
==========================================
+ Hits         9025     9082      +57     
  Misses        159      159              

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[tvdeyen]

I like that feature. Would you mind to fix the remaining specs? We plan to make significant changes to the repo and this will break this PR. Also we plan to release 8.4 soon and we would include in that release

[robinboening]

I like that feature. Would you mind to fix the remaining specs? We plan to make significant changes to the repo and this will break this PR. Also we plan to release 8.4 soon and we would include in that release

Fixed. The expectation was wrong. The dynamically created uncached_layout does not specify cache: 60, so Page#expiration_time correctly falls back to the configured default of 600 seconds.

[tvdeyen]

Nice feature — the no-store reasoning is spot on. 👍 One thing to flag: page_cache_disabled_by_elements? runs on every show/index, and because it calls @page.find_elements, it now loads and instantiates the full published element set on every request — including conditional GETs that return 304. Previously a 304 only ran the lightweight EtagGenerator pluck and never materialized elements, so this adds real work to the hot cache path. It's also evaluated up to 3× per request (twice in set_expiration_headers, once via render_fresh_page?) with no memoization.

Two small things make it nearly free again:

1. Short-circuit statically — in the common case no definition opts out, so we can skip the DB entirely; and when one does, an exists? is far cheaper than loading full records (it also naturally covers nested/fixed elements, which find_elements' default visible.not_nested.unfixed scope currently misses).
2. Memoize per request (guarding with defined? since the result can be false).

def page_cache_disabled_by_elements?
  return @page_cache_disabled_by_elements if defined?(@page_cache_disabled_by_elements)

  opt_out_names = Alchemy::Element.definitions.filter_map { |d| d.name if d.page_cache == false }
  @page_cache_disabled_by_elements = !!(
    opt_out_names.any? &&
      @page&.public_version&.elements&.published&.exists?(name: opt_out_names)
  )
end

This keeps behavior identical for the specs here, stays zero-cost for apps that don't use the opt-out, and closes the nested/fixed-element gap for free. 🙂

performance regression