fix OOM kills: tune MySQL memory, reduce Nextcloud workers, increase Uptime Kuma limit

MySQL (3 OOM kills):
- Cap group_replication_message_cache_size to 128MB (default 1GB caused OOM)
- Reduce innodb_log_buffer_size from 64MB to 16MB
- Lower max_connections from 151 to 80 (peak usage ~40)
- Increase memory limit from 3Gi to 4Gi for headroom

Nextcloud (30+ apache2 OOM kills per incident):
- Reduce MaxRequestWorkers from 50 to 10 to prevent fork bomb
  when SQLite locks cause request pileup
- Lower StartServers/MinSpare/MaxSpare proportionally

Uptime Kuma (Node.js memory leak):
- Increase memory limit from 256Mi to 512Mi
- Increase CPU limit from 200m to 500m

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

stacks/platform/modules/uptime-kuma/main.tf

@@ -71,8 +71,8 @@ resource "kubernetes_deployment" "uptime-kuma" {
               memory = "64Mi"
             }
             limits = {
-              cpu    = "200m"
-              memory = "256Mi"
+              cpu    = "500m"
+              memory = "512Mi"
             }
           }