feat(metadata): add OpenModelDB metadata provider and model source for upscalers

Add OpenModelDB (openmodeldb.info) as a metadata and download source for
the existing upscaler model type.

Metadata:
- New OpenModelDBClient: fetches the site's bulk JSON dumps, caches them
  on disk (24h TTL + ETag revalidation), and builds a local sha256 index
- New OpenModelDBModelMetadataProvider adapts catalogue entries to the
  CivitAI-shaped version dict contract; registered in the fallback chain
  behind the enable_openmodeldb_api setting (default on), gated to the
  upscaler sub-type so other model types never trigger the dump download
- Persisted provenance uses metadata_source "openmodeldb" plus a nested
  openmodeldb block (page URL, architecture, scale, license)

Images: paired-image LR/SR URLs are ephemeral imgdiff.net sessions, so
displayable images come from the site-hosted auto-generated thumbnails
(model-level cover leads images[], per-image thumbs for the rest); the
original comparison URL is kept in meta.comparisonUrl.

Downloads:
- New OpenModelDBSource (flat model ids, omdb: group prefix) with
  resource filename derivation that recovers names hidden mid-path
  (mediafire) or synthesizes {id}.{type} for folder links
- HTML-gateway mirrors (mediafire/mega/drive) are rejected with a clear
  manual-download hint instead of silently saving an HTML page as .pth
- ModelSource base gains is_valid_source_id / default_subdir_parts /
  resolve_download_url hooks so flat-id sources need no platform branches

UI: "View on OpenModelDB" link in the model modal (downloaded and
hash-enriched models), settings toggle next to the CivArchive one.

locales/ru.json

@@ -793,6 +793,8 @@
             "downloadComplete": "Загрузка успешно завершена",
             "enableCivarchiveApi": "Включить CivArchive API как источник метаданных",
             "enableCivarchiveApiHelp": "При включении CivArchive API используется как резервный источник метаданных моделей (например, для моделей, удалённых с CivitAI). Отключите, чтобы полностью избежать ограничений скорости CivArchive.",
+            "enableOpenmodeldbApi": "[TODO: Translate] Enable OpenModelDB as metadata provider",
+            "enableOpenmodeldbApiHelp": "[TODO: Translate] When on, upscaler metadata is also looked up in the OpenModelDB catalogue (openmodeldb.info) when CivitAI has no record. The catalogue is cached locally and refreshed daily.",
             "providerOrder": "Порядок резервных источников метаданных",
             "providerOrderHelp": "CivitAI API всегда проверяется первым. Выберите порядок остальных источников при поиске метаданных.",
             "providerOrderCivitaiArchiveSqlite": "CivitAI → CivArchive → Archive DB",