feat(demo): scheduled demo-account reset (5c)

A security audit of every mutating endpoint found no isolation gaps — all routes
scope by current_user, admin routes are gated, and the demo account is sandboxed
in its own user_id space (can't reach others' data, escalate, or hit admin
routes). The remaining demo concern is communal pollution of its own shared state,
so add an automatic reset: a new 'demo_reset' scheduler job (admin-tunable
interval, default 12h) reuses the manual reset logic, and no-ops without ever
creating a demo account if none exists. Verified: a triggered run wiped the demo's
video states and re-seeded its sample playlists.

frontend/src/i18n/locales/hu/scheduler.json

@@ -76,7 +76,8 @@
     "shorts": "Shorts-besorolás",
     "subscriptions": "Feliratkozások újraszinkronja",
     "playlist_sync": "YouTube lejátszási listák szinkronja",
-    "maintenance": "Karbantartás + ellenőrzés"
+    "maintenance": "Karbantartás + ellenőrzés",
+    "demo_reset": "Demo fiók visszaállítása"
   },
   "queue": {
     "recentPending": "Szinkronra váró csatorna",