Fix authentik postgres version to match data

Pin postgres to 12-alpine to match existing data directory
that was initialized with PostgreSQL 12.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

stacks/authentik/docker-compose.yml

@@ -55,7 +55,7 @@ services:
       - redis
 
   postgresql:
-    image: postgres:15-alpine
+    image: postgres:12-alpine
     container_name: authentik-postgres
     restart: unless-stopped
     environment: