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.

docs/metadata-json-schema.md

@@ -299,9 +299,29 @@ The `metadata_source` field indicates which provider last updated the metadata:
 |-------|--------|
 | `"civitai_api"` | Civitai API |
 | `"civarchive"` | CivArchive API |
+| `"openmodeldb"` | OpenModelDB catalogue (upscaler models only; hash-matched) |
 | `"archive_db"` | Metadata Archive Database |
 | `null` | No external source (user-defined only) |
 
+When `metadata_source` is `"openmodeldb"`, the `civitai` payload is a
+CivitAI-shaped version dict synthesized from the OpenModelDB catalogue entry
+(no numeric `id`/`modelId`), and the OpenModelDB-native details live in its
+`openmodeldb` block (`id`, `url`, `authors`, `architecture`,
+`architectureName`, `scale`, `license`, `date`).
+
+In that payload, `images[].url` is always a displayable asset: paired
+comparisons use the site-hosted thumbnail because the `LR`/`SR` originals
+are ephemeral imgdiff viewer sessions that 404 outside them (the original
+viewer link is kept as `images[].meta.comparisonUrl` for reference), and the
+model-level thumbnail leads the list since the card preview derives from
+`images[0]`. `files[].name` is derived from any URL path segment with a
+model extension (mediafire-style mirrors bury it mid-path) or synthesized as
+`{model_id}.{type}` for folder links.
+
+Models downloaded from OpenModelDB additionally carry `source_platform:
+"openmodeldb"` and `source_url` (the model page URL); their download-time
+hydration is recorded as `metadata_source: "source:openmodeldb"`.
+
 ---
 
 ## Auto-Update Behavior