Add simplified PostgreSQL infrastructure role for database services

- Provides PostgreSQL server as shared database infrastructure
- Follows KISS principle with only essential configuration (11 variables vs 45 originally)
- Implements maximum security with Unix socket-only superuser access
- Uses scram-sha-256 authentication for application users
- Includes SystemD security hardening
- Applications manage their own databases/users via this infrastructure
- Production-ready with data checksums and localhost-only access

site.yml

@@ -10,8 +10,13 @@
   gather_facts: yes
   
   roles:
+    # Infrastructure services
+    - role: postgresql
+      tags: ['postgresql', 'infrastructure', 'database']
     # - role: caddy
     #   tags: ['caddy', 'infrastructure', 'web']
+    
+    # Application services  
     - role: sigvild-gallery
       tags: ['sigvild', 'gallery', 'wedding']