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/core/config.py

@@ -32,7 +32,6 @@ class Settings(BaseSettings):
     mcp_hub_service_token: str = "replace-me-mcp-hub-token"
     mcp_hub_registration_enabled: bool = False
     cors_allow_origins: str = "http://localhost:1228"
-    demo_today: str = "2026-08-01"
 
 
 @lru_cache