v0.5.1

Fixes the upgrade path for vaults whose full-text index had given up under 0.4.0 ("full-text pass was interrupted 3 times" in indexStatus().fts.error, typically caused by one very large note). 0.5.0 handles such notes but left that verdict in place, so the index stayed disarmed. The index version meta is bumped to 4; on the first start after upgrading, the verdict is cleared and a pass is armed, and the pending notes are indexed.