feat(db): add object.fields jsonb column, read it into CatalogueObject

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/domain/src/object.rs

@@ -62,6 +62,8 @@ pub struct CatalogueObject {
     pub recorder: Option<String>,
     pub recording_date: Option<Date>,
     pub visibility: Visibility,
+    /// Flexible field values (field key -> value), validated against the registry.
+    pub fields: serde_json::Value,
     pub created_at: OffsetDateTime,
     pub updated_at: OffsetDateTime,
 }