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/services/test_model_sources.py

@@ -195,6 +195,7 @@ class TestCapabilities:
             "modelscope",
             "modelscope-ai",
             "tensorart",
+            "openmodeldb",
         }
 
     def test_labels_are_brand_names(self):
@@ -202,6 +203,7 @@ class TestCapabilities:
         assert source_label("modelscope") == "ModelScope"
         assert source_label("modelscope-ai") == "ModelScope (International)"
         assert source_label("tensorart") == "TensorArt"
+        assert source_label("openmodeldb") == "OpenModelDB"
         assert source_label("unknown", "fallback") == "fallback"
 
 
@@ -920,7 +922,7 @@ class TestSourceIdValidation:
 class TestDownloadSourceRegistry:
     def test_downloadable_sources_excludes_link_only_sites(self):
         platforms = {source.platform for source in downloadable_sources()}
-        assert platforms == {"huggingface", "modelscope", "modelscope-ai"}
+        assert platforms == {"huggingface", "modelscope", "modelscope-ai", "openmodeldb"}
 
     def test_get_download_source_rejects_link_only_platform(self):
         assert get_download_source("tensorart") is None