Replaces FTS-only /api/search in place. RRF (k=60) fuses ts_rank and
pgvector cosine distance rankings. Vector branch silently skipped when
Ollama times out / errors, keeping search snappy and resilient.
Messages have no embeddings in Plan 3, so they participate in the FTS
branch only.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>