fix: backfill base_path when publication arrives after document
when a document arrives on the firehose before its publication,
it gets indexed with an empty base_path. this adds a reactive
backfill in insertPublication() that patches any such documents,
bumps indexed_at for sync, and clears embedded_at for re-embedding.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>