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

memento-note/prisma/schema.prisma

@@ -302,7 +302,7 @@ model NoteEmbedding {
   noteId    String   @unique
   embedding Unsupported("vector(1536)")
   createdAt DateTime @default(now())
-  updatedAt DateTime @updatedAt
+  updatedAt DateTime @default(now()) @updatedAt
   note      Note     @relation(fields: [noteId], references: [id], onDelete: Cascade)
 
   @@index([noteId])