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

mcp-server/prisma/schema.prisma

@@ -129,11 +129,12 @@ model Note {
 }
 
 model NoteEmbedding {
-  id        String   @id @default(cuid())
-  noteId    String   @unique
-  embedding String
-  createdAt DateTime @default(now())
-  note      Note     @relation(fields: [noteId], references: [id], onDelete: Cascade)
+  id        String                   @id @default(cuid())
+  noteId    String                   @unique
+  embedding Unsupported("vector(1536)")
+  createdAt DateTime                 @default(now())
+  updatedAt DateTime                 @default(now()) @updatedAt
+  note      Note                     @relation(fields: [noteId], references: [id], onDelete: Cascade)
 
   @@index([noteId])
 }