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/demo.py

@@ -99,8 +99,16 @@ def demo_reset(
         actor_label=user.display_name,
         action="demo_reset",
         entity_type="system",
-        metadata={"counts": result.counts},
+        metadata={
+            "counts": result.counts,
+            "anchor_date": result.anchor_date.isoformat(),
+        },
     )
     db.commit()
     response.delete_cookie(settings.session_cookie_name)
-    return {"status": "reset", "counts": result.counts}
+    return {
+        "status": "reset",
+        "counts": result.counts,
+        "anchor_date": result.anchor_date.isoformat(),
+        "seeded_at": result.seeded_at.isoformat(),
+    }