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/registry.py

@@ -14,6 +14,7 @@ from typing import Any, Dict, Mapping, Optional
 from .base import GROUP_PREFIXES, ModelSource, SourceRef, clean_source_url
 from .huggingface import HuggingFaceSource
 from .modelscope import ModelScopeIntlSource, ModelScopeSource
+from .openmodeldb import OpenModelDBSource
 from .tensorart import TensorArtSource
 
 logger = logging.getLogger(__name__)
@@ -26,6 +27,7 @@ _SOURCES: tuple[ModelSource, ...] = (
     ModelScopeSource(),
     ModelScopeIntlSource(),
     TensorArtSource(),
+    OpenModelDBSource(),
 )
 
 _BY_PLATFORM: Dict[str, ModelSource] = {s.platform: s for s in _SOURCES}