`search` fetched, reranked and truncated in one pass, with the reranker's
over-fetch and the reranking itself interleaved in the same branch. Searching
several databases needs to fuse their candidates before anything is ranked, so
the phases have to be separable.
`_fetch` returns one database's candidates, over-fetching only when a reranker
will re-order them. `_rank` orders and cuts them, leaving an image query's vector
ranking alone since there is no text for a reranker to score against. The
over-fetch multiplier is named rather than a literal 10 at the point of use.
Both check the query type before reading `client.reranker`, which is a
cached_property that builds the reranker on first access and loads model weights
for a local one. An image query never used it and must not start.
No behaviour change: the same suite passes, and the search outputs digest
identically to before.
