diff --git a/cloud-init/user-data.yaml.tpl b/cloud-init/user-data.yaml.tpl
index 1bb3b4f..73ef206 100644
--- a/cloud-init/user-data.yaml.tpl
+++ b/cloud-init/user-data.yaml.tpl
@@ -123,6 +123,27 @@ write_files:
       [Install]
       WantedBy=timers.target
 
+  - path: /etc/systemd/system/forgejo-reboot.service
+    content: |
+      [Unit]
+      Description=Apply staged COS updates by rebooting
+
+      [Service]
+      Type=oneshot
+      ExecStart=/sbin/shutdown -r +0
+
+  - path: /etc/systemd/system/forgejo-reboot.timer
+    content: |
+      [Unit]
+      Description=Nightly reboot (lands 30 min after Watchtower so container updates apply first)
+
+      [Timer]
+      OnCalendar=*-*-* 04:30:00
+      Persistent=true
+
+      [Install]
+      WantedBy=timers.target
+
 runcmd:
   - mkdir -p /mnt/disks/forgejo-data
   - if ! blkid /dev/disk/by-id/google-forgejo-data; then mkfs.ext4 -F /dev/disk/by-id/google-forgejo-data; fi
@@ -130,3 +151,4 @@ runcmd:
   - mkdir -p /mnt/disks/forgejo-data/forgejo /mnt/disks/forgejo-data/caddy
   - systemctl enable --now forgejo-stack.service
   - systemctl enable --now forgejo-backup.timer
+  - systemctl enable --now forgejo-reboot.timer
diff --git a/docs/runbook.md b/docs/runbook.md
index 8cef969..c75e3ca 100644
--- a/docs/runbook.md
+++ b/docs/runbook.md
@@ -39,6 +39,30 @@ Single container only:
 docker restart forgejo
 ```
 
+## How updates work
+
+| Layer | Mechanism | Schedule |
+|---|---|---|
+| Host OS (COS) | `cos-update-strategy=update_enabled` stages updates onto the inactive A/B partition; reboot applies them. | Applied on the nightly reboot below. |
+| Forgejo & Caddy patch updates | Watchtower pulls new image digests for the pinned tags (`forgejo:11`, `caddy:2-alpine`). | 04:00 UTC daily (inside the watchtower container; cron `0 0 4 * * *`). |
+| Forgejo major version (e.g. 11→12) | Bump `var.forgejo_image` in tfvars and `terraform apply` — VM is replaced, data disk persists, first boot runs DB migrations. | Manual / deliberate. |
+| Watchtower itself | Pinned at `containrrr/watchtower` (no tag = `latest`), self-updates with `--cleanup`. | 04:00 UTC daily. |
+| Backups | `forgejo-backup.service` via timer. | 03:30 UTC daily. |
+| Reboot to apply COS updates | `forgejo-reboot.service` runs `shutdown -r +0`. Containers come back via `forgejo-stack.service` + `--restart=unless-stopped`. | 04:30 UTC daily. ~30–60s downtime. |
+
+Tonight's order: backup at 03:30 → container update check at 04:00 → reboot at 04:30. Backups always land before any reboot, so a bad update can be rolled back from GCS.
+
+### Disable the nightly reboot
+
+If the reboot ever causes trouble, turn it off without affecting backups or container updates:
+
+```bash
+gcloud compute ssh forgejo --zone=us-east1-b --tunnel-through-iap \
+  --command='sudo systemctl disable --now forgejo-reboot.timer'
+```
+
+Re-enable with `enable --now` instead of `disable --now`. Cloud-init will re-enable it on the next VM replacement regardless.
+
 ## Update containers immediately
 
 Watchtower pulls new images at 04:00 UTC by default. To force now:
@@ -106,5 +130,5 @@ Rotating `SECRET_KEY` invalidates 2FA and some encrypted DB fields. Read the For
 
 ## Cost / billing watch
 
-- Set a project budget alert at $10/month in Cloud Billing (manual; not in Terraform by design — the budget API requires the billing-account-admin role).
+- A $10/month project budget is managed by `terraform/budget.tf`. Email alerts at 50%, 90%, 100% (current spend) and 100% (forecasted) go to `admin_email`. Adjust the threshold via `budget_amount_usd` in tfvars.
 - Skim the billing report monthly. Egress is the most likely surprise.
diff --git a/terraform/budget.tf b/terraform/budget.tf
new file mode 100644
index 0000000..3b31b28
--- /dev/null
+++ b/terraform/budget.tf
@@ -0,0 +1,51 @@
+resource "google_project_service" "billingbudgets" {
+  service            = "billingbudgets.googleapis.com"
+  disable_on_destroy = false
+}
+
+resource "google_monitoring_notification_channel" "email" {
+  display_name = "Forgejo budget alerts"
+  type         = "email"
+  labels = {
+    email_address = var.admin_email
+  }
+}
+
+resource "google_billing_budget" "forgejo" {
+  billing_account = var.billing_account
+  display_name    = "Forgejo project (${var.project_id})"
+
+  budget_filter {
+    projects = ["projects/${var.project_id}"]
+  }
+
+  amount {
+    specified_amount {
+      currency_code = "USD"
+      units         = tostring(var.budget_amount_usd)
+    }
+  }
+
+  threshold_rules {
+    threshold_percent = 0.5
+  }
+  threshold_rules {
+    threshold_percent = 0.9
+  }
+  threshold_rules {
+    threshold_percent = 1.0
+  }
+  threshold_rules {
+    threshold_percent = 1.0
+    spend_basis       = "FORECASTED_SPEND"
+  }
+
+  all_updates_rule {
+    monitoring_notification_channels = [
+      google_monitoring_notification_channel.email.id,
+    ]
+    disable_default_iam_recipients = false
+  }
+
+  depends_on = [google_project_service.billingbudgets]
+}
diff --git a/terraform/variables.tf b/terraform/variables.tf
index 292e2bd..9f3e81d 100644
--- a/terraform/variables.tf
+++ b/terraform/variables.tf
@@ -22,7 +22,18 @@ variable "domain" {
 
 variable "admin_email" {
   type        = string
-  description = "Google account that gets IAP SSH access"
+  description = "Google account that gets IAP SSH access and budget alert emails"
+}
+
+variable "billing_account" {
+  type        = string
+  description = "Billing account ID (format: XXXXXX-XXXXXX-XXXXXX) for the budget alert"
+}
+
+variable "budget_amount_usd" {
+  type        = number
+  default     = 10
+  description = "Monthly budget in USD; alerts fire at 50%, 90%, 100% of this"
 }
 
 variable "forgejo_image" {
diff --git a/terraform/versions.tf b/terraform/versions.tf
index 23190bd..dc87d67 100644
--- a/terraform/versions.tf
+++ b/terraform/versions.tf
@@ -10,7 +10,9 @@ terraform {
 }
 
 provider "google" {
-  project = var.project_id
-  region  = var.region
-  zone    = var.zone
+  project               = var.project_id
+  region                = var.region
+  zone                  = var.zone
+  user_project_override = true
+  billing_project       = var.project_id
 }