refactor(constants): zentrale SW-Timing-Konstanten + minor cleanups

- src/lib/constants.ts: SW_VERSION_QUERY_TIMEOUT_MS, SW_UPDATE_POLL_INTERVAL_MS
- pwa.svelte.ts: nutzt die Konstanten statt 1500/30*60_000
- cache-strategy.ts / diff-manifest.ts: RequestShape/ManifestDiff entkapselt
  (intern statt export, da nirgends extern importiert)
- recipes/[id]/image: deutsche Fehlermeldungen auf Englisch (Konsistenz
  mit allen anderen Endpoints)

Findings aus REVIEW-2026-04-18.md (Quick-Wins 6+7) und dead-code.md

src/lib/client/pwa.svelte.ts

@@ -1,3 +1,5 @@
+import { SW_UPDATE_POLL_INTERVAL_MS, SW_VERSION_QUERY_TIMEOUT_MS } from '$lib/constants';
+
 // Service-Worker-Update-Pattern: Workbox-Style Handshake (kein
 // skipWaiting im install-Handler, User bestätigt via Toast) mit
 // zusätzlichem Zombie-Schutz.
@@ -39,7 +41,7 @@ class PwaStore {
     // mitbekommt, wenn er die Seite lange offen lässt ohne zu navigieren.
     this.pollTimer = setInterval(() => {
       void this.registration?.update().catch(() => {});
-    }, 30 * 60_000);
+    }, SW_UPDATE_POLL_INTERVAL_MS);
   }
 
   private onUpdateFound(): void {
@@ -97,7 +99,7 @@ class PwaStore {
 function queryVersion(sw: ServiceWorker): Promise<string | null> {
   return new Promise((resolve) => {
     const channel = new MessageChannel();
-    const timer = setTimeout(() => resolve(null), 1500);
+    const timer = setTimeout(() => resolve(null), SW_VERSION_QUERY_TIMEOUT_MS);
     channel.port1.onmessage = (e) => {
       clearTimeout(timer);
       const v = (e.data as { version?: unknown } | null)?.version;

src/lib/constants.ts

@@ -0,0 +1,11 @@
+// Shared timing constants. Keep magic numbers here so callers stay readable
+// and the rationale lives next to the value.
+
+// How long to wait for a Service Worker to answer GET_VERSION before
+// treating the response as missing. Short on purpose — SWs that take this
+// long are likely the Chromium zombie case (see pwa.svelte.ts).
+export const SW_VERSION_QUERY_TIMEOUT_MS = 1500;
+
+// Active update check while the page sits open in a tab. 30 minutes is a
+// trade-off between being timely and not hammering the server.
+export const SW_UPDATE_POLL_INTERVAL_MS = 30 * 60_000;

src/lib/sw/cache-strategy.ts

@@ -1,6 +1,6 @@
 export type CacheStrategy = 'shell' | 'swr' | 'images' | 'network-only';
 
-export type RequestShape = { url: string; method: string };
+type RequestShape = { url: string; method: string };
 
 // Pure function — sole decision-maker for "which strategy for this request?".
 // Called by the service worker for every fetch event.

src/lib/sw/diff-manifest.ts

@@ -1,7 +1,7 @@
 // Vergleicht die aktuelle Rezept-ID-Liste (vom Server) mit dem, was
 // der Cache schon hat. Der SW nutzt das Delta, um nur Neue zu laden
 // und Gelöschte abzuräumen.
-export type ManifestDiff = { toAdd: number[]; toRemove: number[] };
+type ManifestDiff = { toAdd: number[]; toRemove: number[] };
 
 export function diffManifest(currentIds: number[], cachedIds: number[]): ManifestDiff {
   const current = new Set(currentIds);

src/routes/api/recipes/[id]/image/+server.ts

@@ -32,13 +32,13 @@ export const POST: RequestHandler = async ({ params, request }) => {
 
   const form = await request.formData().catch(() => null);
   const file = form?.get('file');
-  if (!(file instanceof File)) error(400, { message: 'Feld "file" fehlt' });
-  if (file.size === 0) error(400, { message: 'Leere Datei' });
-  if (file.size > MAX_BYTES) error(413, { message: 'Bild zu groß (max. 10 MB)' });
+  if (!(file instanceof File)) error(400, { message: 'Field "file" missing' });
+  if (file.size === 0) error(400, { message: 'Empty file' });
+  if (file.size > MAX_BYTES) error(413, { message: 'Image too large (max 10 MB)' });
 
   const mime = file.type.toLowerCase();
   const ext = EXT_BY_MIME[mime];
-  if (!ext) error(415, { message: `Bildformat ${file.type || 'unbekannt'} nicht unterstützt` });
+  if (!ext) error(415, { message: `Image format ${file.type || 'unknown'} not supported` });
 
   const buf = Buffer.from(await file.arrayBuffer());
   const hash = createHash('sha256').update(buf).digest('hex');