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/components/modelModal.sourceLinks.test.js

@@ -188,4 +188,41 @@ describe('Model modal source links (#1094)', () => {
     expect(civitaiLink()).toBeNull();
     expect(hfLink()).toBeNull();
   });
+
+  it('renders an OpenModelDB link from the enriched civitai payload', async () => {
+    // Hash-enriched upscalers carry no `source_url`; the page link lives in
+    // `civitai.openmodeldb.url` instead.
+    await renderModal(
+      makeModel({
+        from_civitai: false,
+        civitai: {
+          source: 'openmodeldb',
+          openmodeldb: { id: '4x-UltraSharp', url: 'https://openmodeldb.info/models/4x-UltraSharp' },
+        },
+      })
+    );
+
+    const link = document.querySelector('[data-action="view-model-source"]');
+    expect(link).not.toBeNull();
+    expect(link.dataset.sourceUrl).toBe('https://openmodeldb.info/models/4x-UltraSharp');
+    expect(civitaiLink()).toBeNull();
+  });
+
+  it('prefers source_url over the openmodeldb payload fallback', async () => {
+    await renderModal(
+      makeModel({
+        from_civitai: false,
+        source_url: 'https://openmodeldb.info/models/1x-DeJPG',
+        source_platform: 'openmodeldb',
+        civitai: {
+          source: 'openmodeldb',
+          openmodeldb: { id: '4x-UltraSharp', url: 'https://openmodeldb.info/models/4x-UltraSharp' },
+        },
+      })
+    );
+
+    const links = document.querySelectorAll('[data-action="view-model-source"]');
+    expect(links.length).toBe(1);
+    expect(links[0].dataset.sourceUrl).toBe('https://openmodeldb.info/models/1x-DeJPG');
+  });
 });