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

.gitea/workflows/deploy.yaml

@@ -125,6 +125,7 @@ jobs:
             git reset --hard origin/main
 
             echo "=== Building ==="
+            docker compose pull postgres
             docker compose build memento-note
             docker compose build mcp-server