matrix.archlinux.org: Reduce PostgreSQL memory usage

It ran out of memory, with Postgres using a lot of RSS.

playbooks/matrix.archlinux.org.yml

@@ -14,10 +14,10 @@
     - { role: certbot }
     - { role: nginx }
     - role: postgres
-      postgres_shared_buffers: 1024MB
+      postgres_shared_buffers: 512MB
       postgres_work_mem: 64MB
-      postgres_maintenance_work_mem: 1GB
-      postgres_effective_cache_size: 3GB
+      postgres_maintenance_work_mem: 256MB
+      postgres_effective_cache_size: 1GB
     - role: postfix
       postfix_relayhost: "mail.archlinux.org"
     - { role: matrix }