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.

tests/frontend/utils/modelSourceUrlDetection.test.js

@@ -222,6 +222,17 @@ describe('DownloadManager.detectUrlType — external model source URLs', () => {
         expect(intl.platform).toBe('modelscope-ai');
     });
 
+    it('detects an OpenModelDB model URL with its flat id', () => {
+        const result = DownloadManager.detectUrlType(
+            'https://openmodeldb.info/models/4x-UltraSharp'
+        );
+        expect(result).toEqual({
+            type: 'model-source-repo',
+            platform: 'openmodeldb',
+            repo: '4x-UltraSharp',
+        });
+    });
+
     it('rejects path traversal in either platform', () => {
         expect(
             DownloadManager.detectUrlType('https://modelscope.cn/models/../etc/passwd')