fix: pgvector deployment — idempotent migration, pgvector image, schema sync

- docker-compose.yml: switch postgres:16-alpine to pgvector/pgvector:pg16
- migration: rewrite with IF NOT EXISTS guards, DO block for safe
  text→vector(1536) conversion, handles partial/re-run states
- schema.prisma (both): add @default(now()) on NoteEmbedding.updatedAt,
  sync mcp-server embedding type to Unsupported("vector(1536)")
- deploy.yaml: add docker compose pull postgres before build

docker-compose.yml

@@ -3,7 +3,7 @@ services:
   # PostgreSQL - Shared Database
   # ============================================
   postgres:
-    image: postgres:16-alpine
+    image: pgvector/pgvector:pg16
     container_name: memento-postgres
     restart: unless-stopped
     environment: