syncLabels created Labels with original case (e.g. "Projet") but
searched with .toLowerCase() (e.g. "projet"). On PostgreSQL (case-
sensitive), findMany returned [] → labelRelations disconnected →
orphan cleanup deleted ALL notebook labels.
Fix: use Prisma mode:'insensitive' for findFirst and findMany,
deduplicate case-insensitively while preserving original case.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>