fix: localize dashboard evidence, explain blocked vehicles, clarify pending odometers

Three content defects found by a live reviewer:

- Dashboard attention subtext was raw, untranslated evidence.summary text, and for
  11 of 15 seeded issues that text was literally "Synthetic deterministic seed
  issue". AttentionItem now exposes evidence_signals (stable code + params, same
  shape as the issue detail page) instead of a detail string; the frontend renders
  them through a shared describeEvidenceSignal() used by both the dashboard and the
  issue detail page. Every previously-placeholder seed row now cites a real,
  per-rule-type fact (a genuinely crossed service threshold, a genuinely blank
  field, or a real pair of booking odometer readings) instead of invented prose.

- 5 of 7 blocked vehicles had no quality issue at all and one had only a resolved
  one, so "needs attention" led nowhere. Each now has a real open
  missing_required_field issue backed by a genuinely blank field (no schema change,
  no migration -- reuses the existing data-quality pipeline).

- Booking odometer fields showing a bare "-" for 25 reserved + 1 active booking now
  show a localized explanation ("trip hasn't started yet" / "not yet closed").
  MO-024's rented-but-service-overdue contradiction was already caught by the
  vehicle-status evaluator (DQ-SCAN, vehicle.manual_review_required) -- added a
  regression test rather than new logic.

Also fixed a related bug the above exposed: the vehicle entity_snapshot omitted
registration_number entirely, so the "provide missing fields" form always showed
it blank regardless of the real value.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/tests/test_data_quality.py

@@ -220,6 +220,21 @@ def test_provide_fields_resolves_a_vehicle_missing_field_issue(ops_client):
     assert vehicle["registration_number"] == "TST-999"
 
 
+def test_vehicle_entity_snapshot_includes_registration_number(ops_client):
+    """The snapshot used to omit registration_number entirely, so the 'provide missing
+    fields' form always showed it blank -- even for a vehicle whose plate was actually
+    on file, and even when a *different* field was the genuinely missing one."""
+    issues = ops_client.get(
+        "/api/v1/data-quality/issues",
+        params={"rule_type": "missing_required_field", "status": "open"},
+    ).json()
+    target = next(i for i in issues if i["entity_type"] == "vehicle")
+    vehicle = ops_client.get(f"/api/v1/vehicles/{target['entity_ref']}").json()
+
+    detail = ops_client.get(f"/api/v1/data-quality/issues/{target['public_ref']}").json()
+    assert detail["entity_snapshot"]["registration_number"] == vehicle["registration_number"]
+
+
 def test_resolve_overlap_requires_operations_manager(employee_client):
     response = employee_client.post(
         "/api/v1/data-quality/issues/DQ-DEMO-OVERLAP/resolve-overlap",