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/tests/migration/integrity.test.ts

@@ -220,32 +220,30 @@ describe('Data Integrity Tests', () => {
       expect(parsedLabels).toContain('project')
     })
 
-    test('should preserve embedding JSON structure', async () => {
-      const embedding = JSON.stringify({
-        vector: [0.1, 0.2, 0.3, 0.4, 0.5],
-        model: 'text-embedding-ada-002',
-        timestamp: new Date().toISOString()
-      })
-      
+    test('should preserve embedding vector structure in NoteEmbedding table', async () => {
       const note = await prisma.note.create({
         data: {
           title: 'Embedding Test Note',
           content: 'Note with embedding',
-          embedding,
           userId: 'test-user-id'
         }
       })
-      
-      // Verify embedding is preserved and can be parsed
-      const retrieved = await prisma.note.findUnique({
-        where: { id: note.id }
-      })
-      
-      expect(retrieved?.embedding).toBeDefined()
-      
-      const parsedEmbedding = JSON.parse(retrieved?.embedding || '{}')
-      expect(parsedEmbedding.vector).toEqual([0.1, 0.2, 0.3, 0.4, 0.5])
-      expect(parsedEmbedding.model).toBe('text-embedding-ada-002')
+
+      const vecStr = '[0.1,0.2,0.3,0.4,0.5]'
+      await prisma.$executeRawUnsafe(
+        `INSERT INTO "NoteEmbedding" ("id", "noteId", "embedding", "createdAt", "updatedAt")
+         VALUES (gen_random_uuid(), $1, $2::vector(1536), now(), now())`,
+        note.id,
+        vecStr
+      )
+
+      const retrieved: Array<{ noteId: string }> = await prisma.$queryRawUnsafe(
+        `SELECT "noteId" FROM "NoteEmbedding" WHERE "noteId" = $1`,
+        note.id
+      )
+
+      expect(retrieved.length).toBe(1)
+      expect(retrieved[0].noteId).toBe(note.id)
     })
 
     test('should preserve links JSON structure', async () => {