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>

seed/bookings.csv

@@ -5,10 +5,10 @@ BK-H-0003,CUS-0034,MO-022,2026-07-22T06:00:00Z,2026-07-27T06:00:00Z,returned,314
 BK-H-0004,CUS-0045,MO-029,2026-07-20T05:00:00Z,2026-07-26T05:00:00Z,returned,36708,36912,true
 BK-H-0005,CUS-0056,MO-036,2026-07-18T04:00:00Z,2026-07-25T04:00:00Z,returned,41655,41860,true
 BK-H-0006,CUS-0067,MO-043,2026-07-16T03:00:00Z,2026-07-24T03:00:00Z,returned,46513,46719,true
-BK-H-0007,CUS-0078,MO-050,2026-07-14T02:00:00Z,2026-07-23T02:00:00Z,returned,51685,51892,true
+BK-H-0007,CUS-0078,MO-050,2026-07-14T02:00:00Z,2026-07-23T02:00:00Z,returned,51685,51700,true
 BK-H-0008,CUS-0089,MO-007,2026-07-12T09:00:00Z,2026-07-22T09:00:00Z,returned,20681,20889,true
 BK-H-0009,CUS-0100,MO-014,2026-07-10T08:00:00Z,2026-07-21T08:00:00Z,returned,25419,25628,true
-BK-H-0010,CUS-0111,MO-021,2026-07-18T07:00:00Z,2026-07-20T07:00:00Z,returned,31088,31298,true
+BK-H-0010,CUS-0111,MO-021,2026-07-18T07:00:00Z,2026-07-20T07:00:00Z,returned,31088,31150,true
 BK-H-0011,CUS-0122,MO-028,2026-07-16T06:00:00Z,2026-07-19T06:00:00Z,returned,35948,36159,true
 BK-H-0012,CUS-0133,MO-035,2026-07-14T05:00:00Z,2026-07-18T05:00:00Z,returned,41022,41234,true
 BK-H-0013,CUS-0144,MO-042,2026-07-12T04:00:00Z,2026-07-17T04:00:00Z,returned,46535,46748,true