fix: increase memory for OOMKilled services

- hackmd: 64Mi -> 256Mi (Node.js app OOMKilled after 14min)
- n8n: limit 512Mi -> 768Mi (DB timeouts at 88% mem usage)
- speedtest: 128Mi -> 256Mi (OOMKilled during startup)
- shlink: limit 512Mi -> 768Mi (OOMKilled after startup)

stacks/hackmd/main.tf

@@ -122,10 +122,10 @@ resource "kubernetes_deployment" "hackmd" {
           resources {
             requests = {
               cpu    = "15m"
-              memory = "64Mi"
+              memory = "256Mi"
             }
             limits = {
-              memory = "64Mi"
+              memory = "256Mi"
             }
           }
         }

stacks/n8n/main.tf

@@ -166,7 +166,7 @@ resource "kubernetes_deployment" "n8n" {
               memory = "512Mi"
             }
             limits = {
-              memory = "512Mi"
+              memory = "768Mi"
             }
           }
         }

stacks/speedtest/main.tf

@@ -122,10 +122,10 @@ resource "kubernetes_deployment" "speedtest" {
           resources {
             requests = {
               cpu    = "25m"
-              memory = "128Mi"
+              memory = "256Mi"
             }
             limits = {
-              memory = "128Mi"
+              memory = "256Mi"
             }
           }
           volume_mount {