fix: localize the primary data-quality evidence summary (live-caught on Unraid)

Live validation on the deployed fix branch caught a real bug: every data-quality
issue's top-of-page "Evidence summary" line rendered the raw, always-English legacy
evidence.summary string unconditionally -- in all three languages -- even though the
backend has been emitting structured, localizable evidence.signals for a while
(app/services/data_quality.py already documented this exact intent). The frontend
side of that conversion was never finished.

- DataQualityIssueDetail.tsx now renders evidence.signals through the operator's
  locale as the primary summary; the raw evidence.summary string is only visible
  inside "Technical details" (via the existing EvidenceDisclosure JSON dump).
- The four DQ-DEMO-* seed rows that anchor the guided demo's scripted scenarios now
  carry real, accurate signals computed at seed time (duplicate-customer's similarity
  score is the actual SequenceMatcher ratio on the seeded names, not invented) instead
  of only a legacy English sentence.
- Rows with no structured signals (generic filler seed data) fall back to the raw
  text rather than showing a blank summary; the one known placeholder string gets its
  own localized rendering so it never displays as English filler either.
- New regression test: the vehicle_status_conflict evidence summary must show
  localized text and must never contain the specific raw English sentence that was
  live-visible before this fix, in all 3 languages.

151 backend tests, Ruff, mypy green; full local Playwright suite green (a couple of
sequential-run-only flakes, both confirmed to pass in isolation and unrelated to this
change).

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

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

@@ -101,6 +101,17 @@
         "whyItMatters": "An incorrect status can make an unavailable vehicle appear bookable, or keep an available vehicle hidden from the fleet."
       }
     },
+    "evidence": {
+      "duplicate.exact_email": "Identical email address",
+      "duplicate.exact_phone": "Identical phone number",
+      "duplicate.same_postal_code": "Same postal code",
+      "duplicateSimilarName": "Strongly similar name (score {{score}})",
+      "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."
+    },
     "duplicateCustomer": {
       "heading": "Compare and merge",
       "description": "Choose the canonical customer and review each conflicting field.",