User/Admin saves clear entire Rails cache, causing template cache misses

[philayres]

Problem

On production, browsers refetch /pages/<token>/template more often than expected within the same session, even though app config hasn't changed.

Root cause (confirmed via logs): User and Admin both include AdminHandler, whose after_save :invalidate_cache calls Rails.cache.clear. Since User/Admin are saved on every sign-in (Devise trackable), failed login (lockable), and app-type switch, this wipes the entire shared memcached on nearly every request, destroying Application.server_cache_version and forcing a new template_version token for all users.

Additionally, partial_cache_key (in ApplicationHelper) includes u&.updated_at, so any user-record save changes the cache key even when nothing relevant (roles/app type/access) actually changed — e.g. switching app type A→B→A produces a different key than the original A session.

Plan

1. In AdminHandler#invalidate_cache, keep full Rails.cache.clear for all admin models, but for User/Admin only clear when disabled actually changed (saved_change_to_disabled?); always reset latest_update memo regardless.
2. Remove the u&.updated_at segment from partial_cache_key in ApplicationHelper (role/app-type/access changes are already covered by other segments).
3. Add specs covering: user/admin save without disabled change does not clear cache; disabled change does; other AdminHandler models still clear on every save; partial_cache_key stability across non-relevant user saves.
4. Review existing specs referencing the old clear-on-every-save behavior (admin/app_types_controller_spec.rb, pages_controller_spec.rb, reports_table_helper_spec.rb, memcached_clear_template_recovery_spec.rb).

Longer-term replacement of whole-cache clears with a namespaced/versioned cache-key scheme is out of scope for this issue.