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.

.env.example

@@ -8,7 +8,6 @@ POSTGRES_DB=mobilityops
 POSTGRES_USER=mobilityops
 POSTGRES_PASSWORD=mobilityops
 APP_SECRET=replace-in-production
-DEMO_TODAY=2026-08-01
 TZ=Europe/Brussels
 # Session cookie Secure flag. Keep false for LAN/plain-HTTP deployments (including the
 # current Unraid review environment); set true only once MobilityOps is served over HTTPS,