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/templates/sw.js.j2

@@ -0,0 +1,60 @@
+const CACHE = 'nhl-scoreboard-{{ app_version }}';
+const PRECACHE = {{ precache | tojson }};
+
+self.addEventListener('install', event => {
+    event.waitUntil(caches.open(CACHE).then(c => c.addAll(PRECACHE)));
+    self.skipWaiting();
+});
+
+self.addEventListener('activate', event => {
+    event.waitUntil(
+        caches.keys().then(keys =>
+            Promise.all(keys.filter(k => k !== CACHE).map(k => caches.delete(k)))
+        )
+    );
+    self.clients.claim();
+});
+
+self.addEventListener('fetch', event => {
+    if (event.request.method !== 'GET') return;
+    const { pathname } = new URL(event.request.url);
+
+    // Network-first for the live scoreboard API — stale data is useless
+    if (pathname === '/scoreboard') {
+        event.respondWith(
+            fetch(event.request).catch(() => caches.match(event.request))
+        );
+        return;
+    }
+
+    // Network-first for HTML pages (root, bracket, series detail) so the
+    // very next request after a deploy lands the new asset URLs
+    if (pathname === '/' || pathname === '/bracket' || pathname.startsWith('/series/')) {
+        event.respondWith(
+            fetch(event.request).then(response => {
+                if (response.ok) {
+                    const clone = response.clone();
+                    caches.open(CACHE).then(c => c.put(event.request, clone));
+                }
+                return response;
+            }).catch(() => caches.match(event.request))
+        );
+        return;
+    }
+
+    // Stale-while-revalidate for everything else (versioned static assets,
+    // manifest, icons): return cached bytes immediately, refresh in the
+    // background so the next load is current
+    event.respondWith(
+        caches.match(event.request).then(cached => {
+            const networkFetch = fetch(event.request).then(response => {
+                if (response.ok) {
+                    const clone = response.clone();
+                    caches.open(CACHE).then(c => c.put(event.request, clone));
+                }
+                return response;
+            }).catch(() => cached);
+            return cached || networkFetch;
+        })
+    );
+});