Stability checkpoint — coverage bump (ApiError + Ollama DOWN branch)

Single MR landed since stable-v1.0.40 (svc !176) :

🧪 Tests
- ApiErrorTest (4 tests) — locks the Java record contract for the
  com.mirador.api.ApiError DTO. Was 0% covered ; brings com.mirador.api.*
  package coverage to 100%.
- OllamaHealthIndicatorTest.unexpectedException_returnsDown — tests
  the Health.down() fallback for non-ResourceAccessException errors.
  Bumps OllamaHealthIndicator from 68% → ~75% by exercising the else
  branch of the "is-Ollama-just-not-running" check.

Post-merge main pipeline #770 green on SHA 1b304f6 (MR !176 merge).