feat: let a display survive its network — as code, not as authorization

A display is the one surface expected to outlive its network: a television on
a wall in a building whose uplink is a consumer router, with nobody standing
in front of it. When that uplink drops and the screen reboots, the browser
shows its own error page — not the display shell, not an Ekklesia message,
nothing that recovers on its own, because there is no Ekklesia code left
running to recover.

Cache the shell and the declared code, in a service worker scoped to
`/display/`: the page, the host script, the module's display artifact, and
every contributed renderer that display actually mounted. All of them are code
whose bytes already confer no authority, and all of them are addressed by a
URL carrying an exact version.

Cache nothing else, ever. Not the descriptor, not `/api/v1/principal`, not a
module or platform API response, not the realtime stream, not a pairing
exchange, and not any cross-origin request — which is what a signed
object-delivery URL is. It is an allowlist rather than a deny list, so the
cost of forgetting a new endpoint is that code goes uncached rather than that
a permission decision gets stored.

The consequence is stated rather than papered over: a display cannot render
module content while offline. Mounting needs a descriptor, a descriptor is
authorization-bearing, and it is never cached — so a revoked device stays
revoked and cached code can never manufacture current authorization. What the
cache buys is that a kiosk rebooting mid-outage boots into Ekklesia's own
offline state and keeps retrying, that reconnecting mounts with nothing left
to download, and that a hiccup fetching 600 KB of artifact after the tiny
descriptor already resolved falls back to identical bytes instead of failing
the display.

Network-first everywhere, so a screen picks up new code the moment the server
returns and can never be trapped on an obsolete build — the failure mode a
cache-first worker produces on a screen nobody visits. Only a complete
same-origin 200 is stored. Cache names carry a policy version and superseded
generations are deleted; artifacts are pruned to the current version per
module, keeping every artifact belonging to that version because a display
artifact and its contributed renderers share one. The worker never caches
itself.

The page gets a small signal built from two sources, because either alone
lies: `navigator.onLine` is optimistic behind a captive portal, and only the
worker knows it actually served from cache. A display shows a quiet corner
indicator rather than blanking a lobby wall, and resolves its descriptor again
by itself when the network returns — nobody presses Retry on a lobby TV.

Every caching decision lives in a module that touches no global, so the
behaviour is exercised directly rather than through a worker a test runner
cannot host. Registration is an enhancement: no service workers, plain HTTP,
or a refusing policy all leave a display that works exactly as before.

Co-Authored-By: Claude Opus 5 <[email protected]>

apps/api/src/module-display-routes.ts

@@ -21,6 +21,18 @@ import { moduleDisplayUiArtifactUrl } from "./module-ui-routes.js";
 export const MODULE_DISPLAY_DESCRIPTOR_PREFIX = "/api/v1/module-displays";
 export const MODULE_DISPLAY_PAGE_PREFIX = "/display";
 export const DISPLAY_HOST_SCRIPT_PATH = "/assets/ekklesia-display/app.js";
+/**
+ * Where the display's offline service worker is served (ADR 0043).
+ *
+ * The path is chosen for its **directory**: a service worker's default scope is the directory it was
+ * served from, so `/display/sw.js` controls display pages and nothing else. Serving it from
+ * `/assets/…` beside the host script would scope it to `/assets/`, where no page lives, and widening
+ * it with `Service-Worker-Allowed` would put the Dashboard under a worker it has no use for.
+ *
+ * `sw.js` cannot collide with a display page: a display page is registered at the exact literal
+ * `/display/<module-id>` for each display-capable module, and `sw.js` is not a valid module id.
+ */
+export const DISPLAY_SERVICE_WORKER_PATH = "/display/sw.js";
 
 export function moduleDisplayPageUrl(moduleId: string): string {
   return `${MODULE_DISPLAY_PAGE_PREFIX}/${encodeURIComponent(moduleId)}`;
@@ -107,8 +119,11 @@ function etagMatches(headerValue: string | undefined, etag: string): boolean {
  * bytes with a stale validator. */
 function snapshotAssets(input: DisplayHostAssets): DisplayHostAssets {
   try {
-    const snapshot = createDisplayHostAssets(input.html, input.script);
-    if (snapshot.scriptEtag !== input.scriptEtag) {
+    const snapshot = createDisplayHostAssets(input.html, input.script, input.serviceWorker);
+    if (
+      snapshot.scriptEtag !== input.scriptEtag ||
+      snapshot.serviceWorkerEtag !== input.serviceWorkerEtag
+    ) {
       throw new ModuleDisplayConfigurationError();
     }
     return snapshot;
@@ -252,6 +267,33 @@ export function mountModuleDisplayRoutes(app: Hono, options: ModuleDisplayRoutes
     });
   });
 
+  /**
+   * The worker itself is `no-cache` with a strong validator, exactly like the host script: a browser
+   * re-checks it on its own schedule, and the worker deliberately never caches *itself*, which is
+   * how a screen avoids being trapped on an obsolete worker.
+   *
+   * Like the host script and module artifact bytes, fetching this code grants no authority. It
+   * caches only public code and never a permission decision, so serving it anonymously withholds
+   * nothing the platform was protecting.
+   */
+  app.get(DISPLAY_SERVICE_WORKER_PATH, (c) => {
+    if (etagMatches(c.req.header("if-none-match"), assets.serviceWorkerEtag)) {
+      return new Response(null, {
+        headers: { "Cache-Control": "no-cache", ETag: assets.serviceWorkerEtag },
+        status: 304,
+      });
+    }
+    return new Response(assets.serviceWorker, {
+      headers: {
+        "Cache-Control": "no-cache",
+        "Content-Type": "text/javascript; charset=utf-8",
+        ETag: assets.serviceWorkerEtag,
+        "X-Content-Type-Options": "nosniff",
+      },
+      status: 200,
+    });
+  });
+
   for (const display of displays) {
     mountDisplay(app, { ...options, assets }, display);
   }