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/components/shared/ModelModal.js

@@ -1,5 +1,5 @@
 import { showToast, openCivitai, sendLoraToWorkflow, sendEmbeddingToWorkflow, sendModelPathToWorkflow, buildLoraSyntax, copyToClipboard } from '../../utils/uiHelpers.js';
-import { getModelSourceInfo, getModelSourceGroupKey, getModelSourceViewTitle, openModelSource } from '../../utils/modelSourceHelpers.js';
+import { getModelSource, getModelSourceInfo, getModelSourceGroupKey, getModelSourceViewTitle, openModelSource } from '../../utils/modelSourceHelpers.js';
 import { modalManager } from '../../managers/ModalManager.js';
 import { MODEL_TYPES } from '../../api/apiConfig.js';
 import {
@@ -32,6 +32,18 @@ function getModalFilePath(fallback = '') {
     return fallback;
 }
 
+/**
+ * Source descriptor for a model that was hash-enriched from the OpenModelDB
+ * catalogue: it carries no `source_url`, so `getModelSourceInfo` finds
+ * nothing and the page link lives in the civitai payload instead.
+ */
+function getOpenModelDBSourceInfo(model) {
+    const url = model?.civitai?.openmodeldb?.url;
+    if (typeof url !== 'string' || !url) return null;
+    const descriptor = getModelSource('openmodeldb');
+    return descriptor ? { ...descriptor, sourceId: '', url } : null;
+}
+
 const COMMERCIAL_ICON_CONFIG = [
     {
         key: 'image',
@@ -398,7 +410,7 @@ export async function showModelModal(model, modelType) {
 <div class="civitai-view" title="${translate('modals.model.actions.viewOnCivitai', {}, 'View on Civitai')}" data-action="view-civitai" data-filepath="${escapedFilePathAttr}">
     <i class="fas fa-globe"></i> ${translate('modals.model.actions.viewOnCivitaiText', {}, 'View on Civitai')}
 </div>`.trim() : '';
-    const sourceInfo = getModelSourceInfo(modelWithFullData);
+    const sourceInfo = getModelSourceInfo(modelWithFullData) || getOpenModelDBSourceInfo(modelWithFullData);
     const escapedSourceUrl = sourceInfo?.url ? escapeAttribute(sourceInfo.url) : '';
     const isHuggingFaceSource = sourceInfo?.platform === 'huggingface';
     const sourceTitle = sourceInfo ? getModelSourceViewTitle(sourceInfo) : '';