Single GET /api/search?q=&space_id=&kinds=&limit=&offset= unions FTS
hits across pages / refs / source_docs / messages with a `kind`
discriminator and ts_rank ordering. Each branch's to_tsvector matches
the GIN index expression on its source table so indexes are used.
Messages have no space_id and are excluded when a space filter is set.
Hybrid vector / RRF lands in Plan 3.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
