Harden the analysis re-run trigger: no-JS fallback link, timestamp

[skerbis]

Summary

Stacked on #1061 (the non-blocking analysis page) — please review/merge that one first.

The trigger was only ever a JS-created <button>, with nothing server-rendered at all if JS never ran. During development, a forgotten assets:sync after editing assets/rexstan-analysis.js meant the button silently never appeared - reloading the page didn't help either, since nothing about the trigger was server-side to begin with.

The trigger is now a normal server-rendered link (?rerun=1), so it always renders and works even without JS - clicking it reloads the page, which starts the background run and shows the running state. JS still intercepts the same click to avoid the reload when it did load, and no longer auto-starts a run on the very first page view (previously silent/surprising) in favor of this always-visible link.

Also shows the timestamp the currently displayed result was generated at (RexStanRunStore::getCachedResultTimestamp()), so it's clear when what's on screen isn't from the just-finished run; updates to "just now" client-side once a poll completes.

Test plan

• Verified the link renders correctly and isn't double-escaped (rex_url::backendPage() already returns a pre-escaped URL).
• Verified the no-JS path: clicking reloads the page and starts a background run.
• php -l + PHPStan on the changed files: 0 findings.

🤖 Generated with Claude Code

[skerbis]

Closing as part of flattening the stack into independent PRs — this and #1063/#1064 are being combined into one PR ("Harden, fix and animate the analysis re-run trigger"), based on #1067 instead of chained serially. Will link the replacement here once opened.

[skerbis]

Combined into #1068.