PERF(postgresql): reduce postgresql memory usage

- shared_buffers: 512MB -> 128MB
- effective_cache_size: 1536MB -> 384MB
- memory requests: 512Mi -> 256Mi
- memory limits: 2Gi -> 512Mi

Database size is only 7.5MB, previous settings were excessive.

postgresql/manifests/cluster.yaml

@@ -18,18 +18,18 @@ spec:
   # Resource requests and limits
   resources:
     requests:
-      memory: "512Mi"
-      cpu: "30m"  # Reduced from 150m (actual usage: 9-13m)
+      memory: "256Mi"
+      cpu: "30m"
     limits:
-      memory: "2Gi"
+      memory: "512Mi"
       # cpu: no limit to prevent throttling
 
   # PostgreSQL configuration
   postgresql:
     parameters:
       max_connections: "200"
-      shared_buffers: "512MB"
-      effective_cache_size: "1536MB"
+      shared_buffers: "128MB"
+      effective_cache_size: "384MB"
       maintenance_work_mem: "128MB"
       checkpoint_completion_target: "0.9"
       wal_buffers: "16MB"