fix(demo): anchor seeded dates to the real reset moment

Booking/inspection/maintenance/outbox dates were authored as absolute
timestamps around a fixed 2026-08-01 anchor and never re-anchored at
seed/reset time, so demo scenarios (e.g. BK-DEMO-RETURN) silently drifted
into the past. Every reset now shifts seeded dates by (today - authored
anchor); dashboard's "today" filter uses real wall-clock time instead of
the now-removed frozen demo_today setting. Adds seed-validation tests
proving scenarios S1/S2/S4/S5 are present and internally consistent after
every reset.

backend/app/api/routers/dashboard.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-from datetime import date, datetime
+from datetime import UTC, date, datetime
 from typing import Literal
 
 from fastapi import APIRouter, Depends
@@ -30,7 +30,9 @@ _SEVERITY_ORDER = {"high": 0, "medium": 1, "low": 2}
 
 
 def _today() -> date:
-    return datetime.fromisoformat(settings.demo_today).date()
+    # Seeded dates are shifted to the real reset moment by `seed_loader.py`'s anchor
+    # shift, so "today" must be real wall-clock time, not the frozen `demo_today` setting.
+    return datetime.now(UTC).date()
 
 
 @router.get("", response_model=DashboardOut)