feat: upgrade PostgreSQL to pgvector-enabled image

Changed PostgreSQL image from postgres:16-alpine to
pgvector/pgvector:pg16-alpine to enable vector similarity search
capabilities for AI/RAG applications.

This is required for Open WebUI's RAG functionality to store and
query document embeddings.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

arty.yml

@@ -17,7 +17,7 @@ envs:
     CORE_REDIS_HOST: redis
     CORE_REDIS_PORT: 6379
     CORE_REDIS_IMAGE: redis:7-alpine
-    CORE_POSTGRES_IMAGE: postgres:16-alpine
+    CORE_POSTGRES_IMAGE: pgvector/pgvector:pg16-alpine
     # VPN
     VPN_TRAEFIK_ENABLED: true
     VPN_COMPOSE_PROJECT_NAME: vpn