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.

static/js/utils/modelSourceHelpers.js

@@ -1,5 +1,5 @@
 /**
- * External model source helpers (Hugging Face / ModelScope / TensorArt).
+ * External model source helpers (Hugging Face / ModelScope / TensorArt / OpenModelDB).
  *
  * Mirrors `py/services/model_sources/registry.py` so the frontend and the
  * backend agree on URL recognition, version-group keys, and which sites
@@ -92,6 +92,26 @@ export const MODEL_SOURCES = [
     canonical: (id) => `https://tensor.art/models/${id}`,
     filePage: null,
   },
+  {
+    // Flat catalogue of upscalers: the id IS the published-model identity
+    // (no owner/repo split, no revisions, no per-file page).
+    platform: 'openmodeldb',
+    label: 'OpenModelDB',
+    groupPrefix: 'omdb',
+    groupKey: 'repo',
+    supportsEnrichment: true,
+    supportsDownload: true,
+    defaultRevision: '',
+    defaultSubdir: 'openmodeldb',
+    exampleUrl: 'https://openmodeldb.info/models/4x-UltraSharp',
+    placeholder: 'https://openmodeldb.info/models/4x-UltraSharp',
+    pattern: /^https?:\/\/(?:www\.)?openmodeldb\.info\/models\/([A-Za-z0-9_][A-Za-z0-9_.-]*)/i,
+    filePattern: null,
+    // Flat model ids (no owner/name split).
+    flatId: true,
+    canonical: (id) => `https://openmodeldb.info/models/${id}`,
+    filePage: null,
+  },
 ];
 
 /** Return the source descriptor for a platform id, or null. */
@@ -257,6 +277,18 @@ export function isValidRepoId(repo) {
     .every((part) => part && part !== '.' && part !== '..' && /^[A-Za-z0-9_][\w.-]*$/.test(part));
 }
 
+/**
+ * Source-aware id validation, mirroring `ModelSource.is_valid_source_id`:
+ * `owner/name` for repository sites, a flat token for OpenModelDB.
+ */
+export function isValidSourceId(platform, repo) {
+  const source = getModelSource(platform);
+  if (source?.flatId) {
+    return typeof repo === 'string' && /^[A-Za-z0-9_][A-Za-z0-9_.-]*$/.test(repo);
+  }
+  return isValidRepoId(repo);
+}
+
 /**
  * Recognise a downloadable model-source URL.
  *