Prevents dateString.split crash when Nunjucks | date filter receives
Date objects from MongoDB. Audit timestamps (createdAt, updatedAt,
lastFetchedAt, etc.) now use .toISOString(). Query-used fields
(published, nextFetchAt) kept as Date objects for MongoDB compatibility.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>