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_dashboard.py

@@ -24,6 +24,22 @@ def test_dashboard_attention_items_link_to_records(ops_client):
         assert item["severity"] in ("low", "medium", "high")
 
 
+def test_dashboard_attention_items_expose_localizable_signals_not_raw_text(ops_client):
+    """The dashboard subtext used to be raw, untranslated evidence text (and for most
+    seeded issues, the meaningless placeholder 'Synthetic deterministic seed issue').
+    The API must never emit prose here -- only stable signal codes + params, exactly
+    like the data-quality issue detail page, for the frontend to localize."""
+    response = ops_client.get("/api/v1/dashboard")
+    body = response.json()
+    assert len(body["attention_items"]) > 0
+    for item in body["attention_items"]:
+        assert "detail" not in item
+        assert len(item["evidence_signals"]) > 0
+        for signal in item["evidence_signals"]:
+            assert signal["code"]
+            assert signal["code"] != "Synthetic deterministic seed issue"
+
+
 def test_dashboard_recent_automation_capped_at_five(ops_client):
     response = ops_client.get("/api/v1/dashboard")
     body = response.json()