Auto-generate VAPID keys at startup; no manual configuration needed

On first boot, ensure_vapid_keys() generates a fresh EC P-256 key pair
and saves it to /data/vapid_keys.json (inside the app_data volume) so
it survives container restarts. Subsequent boots load from that file.

VAPID_PRIVATE_KEY / VAPID_PUBLIC_KEY env vars still take precedence for
deployments that prefer to manage keys externally.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/fabledassistant/app.py

@@ -112,10 +112,12 @@ def create_app() -> Quart:
         from fabledassistant.services.llm import ensure_model
         from fabledassistant.services.logging import start_log_retention_loop
         from fabledassistant.services.notifications import start_notification_loop
+        from fabledassistant.services.push import ensure_vapid_keys
 
         start_cleanup_loop()
         start_log_retention_loop()
         start_notification_loop()
+        ensure_vapid_keys()
 
         async def _warm_model(model: str) -> None:
             """Warm an already-installed model into VRAM (no pull)."""