feat(metadata): skip models on offline roots during bulk fetch (#1108)

Bulk metadata refresh picked candidates purely from cache state, so the
entries the scoped scan deliberately keeps for unreachable drives were
fetched anyway — burning CivitAI rate limit on sidecar/preview writes
that cannot land.

- Filter the fetch queue by root reachability (live check via
  describe_model_roots), covering both startup-offline and mid-session
  drive-off; probing failures degrade to no filtering
- Report offline_skipped / offline_roots in the WS terminal payloads,
  the HTTP result and the completion message
- Show a "Drive Offline" stat card in the fetch summary modal when
  any model was skipped this way

locales/ko.json

@@ -2130,6 +2130,7 @@
             "statSuccess": "성공",
             "statFailed": "실패",
             "statSkipped": "건너뜀",
+            "statOffline": "[TODO: Translate] Drive Offline",
             "statTotal": "총 스캔",
             "statDuration": "소요 시간",
             "successMessage": "모든 {count}개 {type}이(가) 성공적으로 업데이트되었습니다",