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>

mcp-server/tools.js

@@ -538,21 +538,25 @@ export function registerTools(server, prisma) {
         }
 
         case 'search_notes': {
-          const where = noteWhere(uid, {
-            isArchived: args.includeArchived || false,
-            OR: [
-              { title: { contains: args.query } },
-              { content: { contains: args.query } },
-            ],
-            ...(args.notebookId ? { notebookId: args.notebookId } : {}),
-          });
+          const safeQuery = (args.query || '').replace(/'/g, "''");
+          const userClause = uid ? `AND "userId" = '${uid}'` : '';
+          const notebookClause = args.notebookId ? `AND "notebookId" = '${args.notebookId.replace(/'/g, "''")}'` : '';
 
-          const notes = await prisma.note.findMany({
-            where,
-            orderBy: [{ isPinned: 'desc' }, { updatedAt: 'desc' }],
-            take: DEFAULT_SEARCH_LIMIT,
-          });
-          return textResult(notes.map(parseNoteLightweight));
+          const ftsRows = await prisma.$queryRawUnsafe(`
+            SELECT id, title, content, color, type, "isPinned", "isArchived",
+                   "isMarkdown", size, "createdAt", "updatedAt", "notebookId",
+                   images, labels, "checkItems", reminder, "isReminderDone"
+            FROM "Note"
+            WHERE "tsv" @@ plainto_tsquery('simple', '${safeQuery}')
+              AND "trashedAt" IS NULL
+              AND "isArchived" = ${args.includeArchived ? 'true' : 'false'}
+              ${userClause}
+              ${notebookClause}
+            ORDER BY ts_rank("tsv", plainto_tsquery('simple', '${safeQuery}')) DESC
+            LIMIT ${DEFAULT_SEARCH_LIMIT}
+          `);
+
+          return textResult(ftsRows.map(parseNoteLightweight));
         }
 
         case 'move_note': {