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.

py/services/model_sources/__init__.py

@@ -1,4 +1,4 @@
-"""External model-source providers (Hugging Face, ModelScope, TensorArt).
+"""External model-source providers (Hugging Face, ModelScope, TensorArt, OpenModelDB).
 
 This package is the single abstraction over "a site that hosts models and
 a model card".  See :mod:`py.services.model_sources.base` for the provider
@@ -30,6 +30,7 @@ from .hydration import (
     resolve_site_base_model,
 )
 from .modelscope import ModelScopeIntlSource, ModelScopeSource
+from .openmodeldb import OpenModelDBSource
 from .registry import (
     LEGACY_HF_URL_FIELD,
     SOURCE_PLATFORM_FIELD,
@@ -59,6 +60,7 @@ __all__ = [
     "HuggingFaceSource",
     "ModelScopeIntlSource",
     "ModelScopeSource",
+    "OpenModelDBSource",
     "SOURCE_PLATFORM_FIELD",
     "SOURCE_URL_FIELD",
     "SourceRef",