Add ErrorCode.INVALID_DATE_RANGE and a validateDateRange guard on
DocumentService.updateDocument, run right after applyDatePrecision so it
fires before any save (updateDocumentTags persists earlier in the method).
Mirrors the V69 chk_meta_date_end_after_start CHECK: end >= start with a
null start allowed, using isBefore so equal dates stay valid. Turns a user
date typo into a clean 400 instead of a 500 + Sentry alert.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>