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_metadata_provider.py

@@ -112,7 +112,10 @@ class _RateLimitRetryHelper:
 
 # Labels of providers that are free to consult even while a network provider
 # is rate-limited (local lookups, no vendor cost).
-_LOCAL_PROVIDER_LABELS = frozenset({"sqlite"})
+# "openmodeldb_api" qualifies because its lookups hit a local index built from
+# a cached bulk dump; the underlying site is a static host (GitHub Pages), so
+# even a cold cache refresh is a single cheap GET against a different vendor.
+_LOCAL_PROVIDER_LABELS = frozenset({"sqlite", "openmodeldb_api"})
 
 
 class ModelMetadataProvider(ABC):
@@ -480,6 +483,36 @@ class SQLiteModelMetadataProvider(ModelMetadataProvider):
         except json.JSONDecodeError:
             return None
         
+class OpenModelDBModelMetadataProvider(ModelMetadataProvider):
+    """Provider that serves upscaler metadata from the OpenModelDB catalogue.
+
+    Only hash lookups are supported: OpenModelDB has no per-model or version
+    API, so the remaining provider surface intentionally returns None and lets
+    the fallback chain continue to the next provider.
+    """
+
+    def __init__(self, openmodeldb_client):
+        self.client = openmodeldb_client
+
+    async def get_model_by_hash(self, model_hash: str) -> Tuple[Optional[Dict[str, Any]], Optional[str]]:
+        return await self.client.get_model_by_hash(model_hash)
+
+    async def get_model_versions(self, model_id: str) -> Optional[Dict[str, Any]]:
+        """Not supported: OpenModelDB models have no version history API."""
+        return None
+
+    async def get_model_version(self, model_id: Optional[int] = None, version_id: Optional[int] = None) -> Optional[Dict[str, Any]]:
+        """Not supported: OpenModelDB models have no version history API."""
+        return None
+
+    async def get_model_version_info(self, version_id: str) -> Tuple[Optional[Dict[str, Any]], Optional[str]]:
+        """Not supported: OpenModelDB models have no version history API."""
+        return None, "Model not found"
+
+    async def get_user_models(self, username: str, cursor: Optional[str] = None) -> Optional[Dict[str, Any]]:
+        """Not supported by the OpenModelDB provider."""
+        return None
+
 class FallbackMetadataProvider(ModelMetadataProvider):
     """Try providers in order, return first successful result.
 
@@ -750,6 +783,32 @@ class FallbackMetadataProvider(ModelMetadataProvider):
     def _iter_providers(self):
         return zip(self.providers, self._provider_labels)
 
+    def excluding(self, labels: "frozenset[str] | set[str]") -> "FallbackMetadataProvider":
+        """Return a copy of this chain without the providers named in *labels*.
+
+        Used by the metadata sync service to skip providers that cannot apply
+        to a given model (e.g. OpenModelDB only indexes upscalers), so their
+        cold-start cost (a bulk dump download) is never paid pointlessly.
+        """
+        kept = [
+            (label, provider)
+            for provider, label in self._iter_providers()
+            if label not in labels
+        ]
+        if len(kept) == len(self.providers):
+            return self
+        if not kept:
+            # Never produce an empty chain; the caller still needs a provider
+            # that can at least report "Model not found".
+            return self
+        return FallbackMetadataProvider(
+            kept,
+            rate_limit_retry_limit=self._rate_limit_retry_limit,
+            rate_limit_base_delay=self._rate_limit_base_delay,
+            rate_limit_max_delay=self._rate_limit_max_delay,
+            rate_limit_jitter_ratio=self._rate_limit_jitter_ratio,
+        )
+
     async def _call_with_rate_limit(self, label: str, func, *args, **kwargs):
         return await self._rate_limit_helper.run(label, func, *args, **kwargs)