feat(search): make hybrid search the UI default with a Vector/Hybrid toggle

[netclectic]

Summary

Make the web UI search use hybrid search (vector + PostgreSQL full-text via Reciprocal Rank Fusion) by default, as documented in ADR docs/adr/2026-02-14-hybrid-search-rrf.md, and add a minimum-similarity control.

Changes

• GET /api/memory/search now defaults to HybridSearch via a method=hybrid|vector query param; method=vector keeps the metadata-embedding path.
• Hybrid results are post-filtered by vector similarity when a threshold is supplied. Default threshold 0.25 (pass 0 to disable). Full-text-only matches (which have no similarity score) are excluded when a threshold is active.
• The HybridSearch storage method is unchanged (still ignores minSimilarity per the ADR), so the MCP SearchMemories tool and short-keyword searches keep working.
• UI: a Hybrid / Vector toggle plus a 0–100 step-5 slider for the threshold, with mode-aware defaults (Hybrid 25%, Vector 70%). The sidebar search benefits automatically.

Note: this branch builds on #201 (workspace-page) and its diff includes those changes until #201 merges.

Related

• Use HybridSearch as the default UI search with a Vector/Hybrid toggle netclectic/memorizer#3