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/managers/DownloadManager.js

@@ -18,7 +18,7 @@ import {
     detectModelSourceDownloadUrl,
     getModelSource,
     isExternalModelSource,
-    isValidRepoId,
+    isValidSourceId,
 } from '../utils/modelSourceHelpers.js';
 
 export class DownloadManager {
@@ -534,7 +534,7 @@ export class DownloadManager {
         const sourceInfo = detectModelSourceDownloadUrl(trimmed);
         if (sourceInfo) {
             // Reject path-traversal patterns like "../.." or "user/.."
-            if (!isValidRepoId(sourceInfo.repo)) {
+            if (!isValidSourceId(sourceInfo.platform, sourceInfo.repo)) {
                 return null;
             }
             return {