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.

backend/app/config.py

@@ -94,6 +94,8 @@ class Settings(BaseSettings):
     autotag_interval_minutes: int = 30
     subscriptions_resync_minutes: int = 360
     playlist_sync_minutes: int = 360
+    # How often the shared demo account is auto-reset to a clean baseline (communal sandbox).
+    demo_reset_minutes: int = 720
     # --- Maintenance / validation job ---
     # Runs once a day by default (admin-tunable via the Scheduler dashboard). Grace periods
     # are in days because that's the granularity that matters; the rolling re-validation