Use Kubernetes pod creation timestamp for deploy time in footer

- Read DEPLOY_TIMESTAMP environment variable for actual deploy time
- Fall back to compile time in development when env var not set
- Add Kubernetes Downward API to inject metadata.creationTimestamp
- Update k8s/deployment.yaml to set DEPLOY_TIMESTAMP env var
- Document deployment timestamp mechanism in k8s/README.md

This ensures the footer shows when the pod was deployed to the cluster,
not when the Docker image was built in CI/CD.

k8s/README.md

@@ -12,6 +12,20 @@ Required secrets in the `towerops` namespace:
 
 For local development, the project root `.envrc` is used by direnv.
 
+## Deployment Timestamp
+
+The application footer displays the deployment timestamp to track when the current version was deployed. This is automatically set using the Kubernetes Downward API:
+
+```yaml
+env:
+  - name: DEPLOY_TIMESTAMP
+    valueFrom:
+      fieldRef:
+        fieldPath: metadata.creationTimestamp
+```
+
+The pod's creation timestamp is injected as an environment variable and displayed in the footer as "Last deployed X ago · YYYY-MM-DD HH:MM:SS UTC".
+
 ## Deploying
 
 Apply all resources using kustomize:

k8s/deployment.yaml

@@ -55,6 +55,10 @@ spec:
               valueFrom:
                 fieldRef:
                   fieldPath: metadata.namespace
+            - name: DEPLOY_TIMESTAMP
+              valueFrom:
+                fieldRef:
+                  fieldPath: metadata.creationTimestamp
             - name: RELEASE_DISTRIBUTION
               value: "name"
             - name: RELEASE_NODE

lib/towerops/application.ex

@@ -5,7 +5,7 @@ defmodule Towerops.Application do
 
   use Application
 
-  # Capture the build/deploy timestamp at compile time
+  # Capture the build timestamp at compile time (fallback for development)
   @build_timestamp DateTime.utc_now()
 
   @impl true
@@ -49,11 +49,41 @@ defmodule Towerops.Application do
   end
 
   @doc """
-  Returns the build/deploy timestamp.
+  Returns the deployment timestamp.
 
-  This timestamp is captured at compile time and represents when the
-  application was last built/deployed.
+  In production (Kubernetes), this reads from the DEPLOY_TIMESTAMP environment
+  variable which should be set during deployment. Falls back to compile time
+  in development.
+
+  Set DEPLOY_TIMESTAMP in your Kubernetes deployment:
+  ```yaml
+  env:
+    - name: DEPLOY_TIMESTAMP
+      value: "2026-01-17T12:34:56Z"
+  ```
+
+  Or use a downward API to inject the pod creation time:
+  ```yaml
+  env:
+    - name: DEPLOY_TIMESTAMP
+      valueFrom:
+        fieldRef:
+          fieldPath: metadata.creationTimestamp
+  ```
   """
   @spec build_timestamp() :: DateTime.t()
-  def build_timestamp, do: @build_timestamp
+  def build_timestamp do
+    case System.get_env("DEPLOY_TIMESTAMP") do
+      nil ->
+        # Development/fallback: use compile time
+        @build_timestamp
+
+      timestamp_string ->
+        # Production: parse from environment variable
+        case DateTime.from_iso8601(timestamp_string) do
+          {:ok, datetime, _offset} -> datetime
+          {:error, _} -> @build_timestamp
+        end
+    end
+  end
 end