feat: migrate semantic search to pgvector + full-text search

Replace JSON-string embeddings with native pgvector(1536) storage and
add PostgreSQL full-text search (tsvector/GIN) with Reciprocal Rank Fusion
for hybrid keyword + semantic ranking.

Changes:
- NoteEmbedding.embedding: String → vector(1536) via pgvector
- NoteEmbedding: added updatedAt for reindex tracking
- Note: added tsv (tsvector) with auto-update trigger for FTS
- semantic-search.service: hybrid FTS + vector search with RRF fusion
- embedding.service: toVectorString() for pgvector SQL literals
- Removed JS-side cosine similarity loops (now DB-side via <=>)
- Added HNSW index on NoteEmbedding.embedding (cosine distance)
- Added GIN index on Note.tsv for FTS queries

Schema migration in: prisma/migrations/20260512120000_pgvector_and_fts_search/

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

memento-note/components/home-client.tsx

@@ -259,7 +259,6 @@ export function HomeClient({ initialNotes, initialSettings }: HomeClientProps) {
     const labelFilter = searchParams.get('labels')?.split(',').filter(Boolean) || []
     const colorFilter = searchParams.get('color')
     const notebook = searchParams.get('notebook')
-    const semanticMode = searchParams.get('semantic') === 'true'
 
     const isBackgroundRefresh = refreshKey > prevRefreshKey.current
     prevRefreshKey.current = refreshKey
@@ -271,7 +270,7 @@ export function HomeClient({ initialNotes, initialSettings }: HomeClientProps) {
         setIsLoading(true)
       }
       let allNotes = search
-        ? await searchNotes(search, semanticMode, notebook || undefined)
+        ? await searchNotes(search, true, notebook || undefined)
         : await getAllNotes(false, notebook || undefined)
 
       const sharedOnly = searchParams.get('shared') === '1'