feat: cache-control overhaul so visual changes propagate immediately

Per-file content-hash versioning on every /static reference, immutable cache
headers on versioned URLs, no-cache on HTML, auto-bumped service worker cache
name with stale-while-revalidate for assets, and a controllerchange listener
that silently reloads the page when a new SW takes control.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/static/script.js

@@ -417,5 +417,11 @@ window.addEventListener('load', () => {
         navigator.serviceWorker.register('/sw.js').catch(err => {
             console.warn('Service worker registration failed:', err);
         });
+        let reloading = false;
+        navigator.serviceWorker.addEventListener('controllerchange', () => {
+            if (reloading) return;
+            reloading = true;
+            location.reload();
+        });
     }
 });