Use HybridSearch as the default UI search with a Vector/Hybrid toggle

[netclectic]

Summary

The UI search endpoint /api/memory/search currently uses SearchWithMetadataEmbedding, which fails to return results for short keyword queries (e.g., "race condition", "phobos"). The root cause and fix are documented in ADR docs/adr/2026-02-14-hybrid-search-rrf.md: cosine-similarity compression makes the default 0.7 threshold eliminate genuinely relevant content, while HybridSearch (vector + PostgreSQL FTS + Reciprocal Rank Fusion) resolves it — it's already the default in the MCP SearchMemories tool but was never wired into the web UI.

Changes

Backend (src/Memorizer/Controllers/MemoryController.cs)

• SearchMemories (GET /api/memory/search) gains a [FromQuery] string? method = "hybrid" parameter.
• hybrid (default) → _storage.HybridSearch(...); vector → existing SearchWithMetadataEmbedding. No storage-layer changes needed — both methods already exist on IStorage with identical signatures.

Frontend (src/Memorizer/Views/Home/Index.cshtml)

• Add a Hybrid / Vector mode toggle to the search bar (Hybrid is the default).
• Hybrid mode: placeholder "Search memories...", button "Search", hide the "Min similarity (0-1)" field (a no-op for HybridSearch per the ADR).
• Vector mode: show the min-similarity field and the "Vector Search" button.
• Rename runVectorSearch() → runSearch() passing method=. Relabel the results header.
• The sidebar search (/memories?q=...) routes through the same code path and benefits automatically.

Notes

• No dependencies.
• minSimilarity is accepted but not applied by HybridSearch (by design); the parameter is retained for API compatibility.

[netclectic]

Implemented on branch feature/hybrid-search-ui (pushed to this fork, stacked on feature/workspace-page).

• GET /api/memory/search now defaults to HybridSearch via a method=hybrid|vector query param; method=vector keeps the metadata-embedding path.
• Min-similarity filter for hybrid mode: hybrid results are post-filtered on vector similarity when a threshold is provided; default is 0.25 (pass 0 to disable). Full-text-only matches (no similarity score) are excluded when a threshold is active.
• The HybridSearch storage method is unchanged (still ignores minSimilarity per ADR 2026-02-14), so MCP SearchMemories behavior and short-keyword searches are unaffected.
• UI: search bar with a Hybrid / Vector toggle and a 0–100 step-5 slider for the threshold. Mode-aware defaults (Hybrid = 25%, Vector = 70%) that reset when switching modes. Sidebar search benefits automatically.

Follow-up refinements applied after review: min-sim slider defaults per mode, default threshold 0.25.

Tests: 195 unit + 188 integration pass on the branch (includes search-dispatch tests updated for the hybrid default + new hybrid threshold tests).

[netclectic]

Opened upstream PR: petabridge#202 (draft - stacked on petabridge#201).