After a pod restart, in-memory indexes (universe bitmap, category,
format, creator, tags, duration, created_at) were empty because they
were only populated by write_item_with_metadata() calls. This caused
/health to report items: 0 and queries to return no results despite
data persisting on disk in fjall storage.
Add rebuild_item_indexes() which scans the items keyspace for Tag::Meta
entries on startup and repopulates all indexes from stored metadata.
Update m2_uat crash recovery test to assert indexes survive restart
without rewriting items.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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