feat(infra): gate observability stack behind var.enable_monitoring (#38)

Adds enable_monitoring variable (default true) that controls whether
Prometheus/Grafana, Loki, Fluent Bit, and Jaeger are deployed.
Setting it to false saves ~1.5 GB RAM, making the stack viable on
a 2–4 GB VPS without touching the application services.

Also caps MongoDB WiredTiger cache at 256 MB (--wiredTigerCacheSizeGB=0.25)
so it doesn't balloon on memory-constrained hosts.

Co-authored-by: Gonçalo Rodrigues <guga@Goncalos-MacBook-Pro.local>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

infrastructure/terraform/mongodb.tf

@@ -77,6 +77,7 @@ resource "kubernetes_stateful_set" "mongodb" {
         container {
           name  = "mongodb"
           image = "mongo:8"
+          args  = ["--wiredTigerCacheSizeGB=0.25"]
 
           env {
             name  = "MONGO_INITDB_ROOT_USERNAME"

infrastructure/terraform/monitoring.tf

@@ -1,9 +1,11 @@
 resource "random_password" "grafana" {
+  count   = var.enable_monitoring ? 1 : 0
   length  = 24
   special = false
 }
 
 resource "helm_release" "kube_prometheus_stack" {
+  count      = var.enable_monitoring ? 1 : 0
   name       = "kps"
   namespace  = kubernetes_namespace.domains["monitoring"].metadata[0].name
   repository = "https://prometheus-community.github.io/helm-charts"
@@ -16,7 +18,7 @@ resource "helm_release" "kube_prometheus_stack" {
       enabled = false
     }
     grafana = {
-      adminPassword = random_password.grafana.result
+      adminPassword = random_password.grafana[0].result
       ingress = {
         enabled          = true
         hosts            = ["grafana.homelab.local"]
@@ -78,6 +80,7 @@ resource "helm_release" "kube_prometheus_stack" {
 }
 
 resource "helm_release" "jaeger" {
+  count      = var.enable_monitoring ? 1 : 0
   name       = "jaeger"
   namespace  = kubernetes_namespace.domains["monitoring"].metadata[0].name
   repository = "https://jaegertracing.github.io/helm-charts"
@@ -99,6 +102,7 @@ resource "helm_release" "jaeger" {
 }
 
 resource "helm_release" "loki" {
+  count      = var.enable_monitoring ? 1 : 0
   name       = "loki"
   namespace  = kubernetes_namespace.domains["monitoring"].metadata[0].name
   repository = "https://grafana.github.io/helm-charts"
@@ -175,6 +179,7 @@ resource "helm_release" "loki" {
 }
 
 resource "helm_release" "fluent_bit" {
+  count      = var.enable_monitoring ? 1 : 0
   name       = "fluent-bit"
   namespace  = kubernetes_namespace.domains["monitoring"].metadata[0].name
   repository = "https://fluent.github.io/helm-charts"

infrastructure/terraform/variables.tf

@@ -3,3 +3,9 @@ variable "enable_gitea" {
   type        = bool
   default     = false
 }
+
+variable "enable_monitoring" {
+  description = "Deploy Prometheus, Grafana, Loki, Jaeger, and Fluent Bit. Set to false on small VMs to save ~1.5 GB RAM."
+  type        = bool
+  default     = true
+}