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.

docs/13-seed-and-demo-scenarios.md

@@ -42,6 +42,19 @@ One seeded outbox/workflow record is failed with a safe simulated connection err
 
 Question: “What must I do when a vehicle returns with damage?” Expected: answer cites damage handling and return inspection procedures.
 
+## Date anchoring
+
+The committed CSVs store absolute ISO timestamps authored around a fixed anchor date
+(`SEED_AUTHORED_ANCHOR = 2026-08-01` in `backend/app/seed_loader.py`, matching the
+`--anchor` used to generate them). Every seed/reset shifts every seeded booking,
+inspection, maintenance and outbox timestamp by `today − SEED_AUTHORED_ANCHOR`, so
+"today"/"near-future"/"currently overlapping" scenarios stay true to the real moment the
+environment was (re)seeded instead of decaying as real time passes between resets. Public
+refs and entity relationships are untouched by the shift — only datetime columns move.
+`load_seed()` returns the resolved `anchor_date`/`seeded_at`, and records a
+`demo_data_seeded` audit event carrying both the resolved anchor and the original
+authoring anchor, so the shift applied on any given reset stays traceable.
+
 ## Demo reset
 
 Reset must:
@@ -49,6 +62,7 @@ Reset must:
 - require Operations Manager;
 - rebuild the deterministic dataset;
 - re-establish scenario references;
+- re-anchor scenario dates to the real reset moment (see above);
 - clear non-seed audit/workflow state;
 - complete safely and visibly;
 - be covered by a test.