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.

compose.yaml

@@ -24,7 +24,6 @@ services:
       DATABASE_URL: ${DATABASE_URL:-postgresql+psycopg://mobilityops:mobilityops@db:5432/mobilityops}
       TZ: ${TZ:-Europe/Brussels}
       APP_SECRET: ${APP_SECRET:-replace-in-production}
-      DEMO_TODAY: ${DEMO_TODAY:-2026-08-01}
       CORS_ALLOW_ORIGINS: ${MOBILITYOPS_PUBLIC_URL:-http://localhost:1228}
       KNOWLEDGE_PROVIDER: ${KNOWLEDGE_PROVIDER:-demo}
       RAGCORE_BASE_URL: ${RAGCORE_BASE_URL:-http://ragcore-api:8000}