feat(metadata): allow manual search with providers outside field rules

Previously, the explicit metadata search modal for a book was restricted
to only the providers configured in the library's field rules. This
prevented users from manually searching a specific provider if it
was not part of their default automated fetch pipeline.

This change allows all globally enabled providers to be used during
a manual metadata search, while still defaulting the selected UI
checkboxes to the providers specified in the field rules.

packages/types/src/metadata-fetch.ts

@@ -78,6 +78,7 @@ export interface MetadataProviderInfo {
   key: MetadataProviderKey;
   label: string;
   identifiable: boolean;
+  selectedByFieldRules?: boolean;
 }
 
 export type MetadataFetchEmptyReason = "no_active_providers" | "providers_throttled" | "no_candidates" | "no_resolved_fields";