Migrate the search index in place instead of a full Tika re-crawl when the shared
search.SchemaRevision is bumped.
bleve (single instance): rebuild from the existing documents into a temp index and
atomically swap, with crash recovery; auto-migrate on startup (opt-out via
SEARCH_ENGINE_BLEVE_AUTO_MIGRATE).
OpenSearch (scaled): versioned indices <base>-v<rev> (no alias), filled from the
newest older index via a create-only scroll reindex - idempotent, never overwrites,
safe to run live. 'opencloud search index migrate' fills the current-revision index;
'opencloud search index prune' drops older ones. Per-document fixups are gated on the
source schema version.
Trash preserved, geopoint siblings synthesized, all fields round-tripped.
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