fix(white-page): Service Worker Kill-Switch + Cache-Control no-cache für index.html

- /sw.js und /service-worker.js liefern jetzt einen Self-Unregister Service Worker
- index.html bekommt Cache-Control: no-cache, no-store, must-revalidate
- Fixt das wiederkehrende weiße-Seite-Problem nach Deploys

app/main.py

@@ -14,7 +14,7 @@ from contextlib import asynccontextmanager
 import structlog
 from fastapi import Depends, FastAPI, HTTPException, Request
 from fastapi.middleware.cors import CORSMiddleware
-from fastapi.responses import FileResponse, JSONResponse
+from fastapi.responses import FileResponse, JSONResponse, PlainTextResponse
 from fastapi.staticfiles import StaticFiles
 from starlette.middleware.base import BaseHTTPMiddleware
 
@@ -639,9 +639,34 @@ def create_app() -> FastAPI:
             blocked_prefixes = ("var/log/", "error/", "error_log", "var/", "etc/", "proc/", "sys/")
             if full_path.startswith(blocked_prefixes) or ".." in full_path:
                 raise HTTPException(status_code=404, detail="Not Found")
+
+            # Kill switch for old PWA service workers — return self-unregistering SW
+            if full_path in ("sw.js", "service-worker.js"):
+                return PlainTextResponse(
+                    content="""// Kill switch — unregister all service workers
+self.addEventListener('install', (e) => { self.skipWaiting(); });
+self.addEventListener('activate', (e) => {
+  e.waitUntil(
+    self.registration.unregister().then(() => {
+      console.log('Service Worker unregistered');
+      return self.clients.claim();
+    })
+  );
+});
+self.addEventListener('fetch', (e) => {
+  e.respondWith(fetch(e.request).catch(() => new Response('', {status: 504})));
+});
+""",
+                    media_type="application/javascript",
+                    headers={"Cache-Control": "no-cache, no-store, must-revalidate"},
+                )
+
             index_path = os.path.join(frontend_dist, "index.html")
             if os.path.isfile(index_path):  # noqa: ASYNC240
-                return FileResponse(index_path)
+                return FileResponse(
+                    index_path,
+                    headers={"Cache-Control": "no-cache, no-store, must-revalidate"},
+                )
             raise HTTPException(status_code=404, detail="Frontend not built")
 
     return app