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>

frontend/src/i18n/locales/en-GB/quality.json

@@ -109,8 +109,7 @@
       "missingField": "Missing field: {{field}}",
       "overlapReservedBookings": "Overlapping bookings: {{refs}}",
       "odometerRegression": "Booking {{laterRef}} recorded {{laterKm}} km, below the {{earlierKm}} km recorded by earlier booking {{earlierRef}}.",
-      "upcomingBookingMissingInspection": "Booking {{bookingRef}} starts soon but the required operational inspection is still missing.",
-      "seedPlaceholder": "Synthetic seed data with no further detail."
+      "upcomingBookingMissingInspection": "Booking {{bookingRef}} starts soon but the required operational inspection is still missing."
     },
     "duplicateCustomer": {
       "heading": "Compare and merge",