add Flatcar lifecycle management backlog

TODO.md

@@ -94,6 +94,32 @@ committed to Git, stored in Terraform state, or printed in CI logs.
 - [ ] Document which Proxmox actions remain outside Ansible, such as selecting
   storage, VM ID, bridge, and Flatcar image source.
 
+## Priority 4a: Flatcar lifecycle management guide
+
+- [ ] Write a dedicated Flatcar lifecycle guide under `docs/`.
+  - [ ] Explain Flatcar release channels and how to select a channel for test,
+    staging, and production hosts.
+  - [ ] Document how to pin and record the Flatcar image or release version.
+  - [ ] Document Ignition as first-boot configuration and explain which host
+    changes belong in Ignition versus Ansible.
+  - [ ] Document update-engine behavior, update deferral, maintenance windows,
+    and controlled reboots.
+  - [ ] Add pre-update health checks for Kubernetes nodes, Falco, Cilium,
+    container runtime access, storage, and kubeconfig connectivity.
+  - [ ] Add a drain and uncordon workflow for Kubernetes nodes before and after
+    a reboot.
+  - [ ] Document how to update Proxmox Flatcar images without overwriting the
+    VM disk before the replacement image is validated.
+  - [ ] Document cloud image or AMI replacement and instance rotation for the
+    Terraform path.
+  - [ ] Define rollback procedures for failed updates, including the previous
+    Flatcar image, VM snapshot policy, and Kubernetes workload recovery.
+  - [ ] Define recovery steps for failed Ignition, failed boot, lost SSH, and
+    a node that does not return to `Ready`.
+  - [ ] Document backup and recovery requirements for kubeconfig, GitOps state,
+    Ansible Vault data, and any locally stored cluster credentials.
+  - [ ] Add a lifecycle checklist suitable for interviews and portfolio review.
+
 ## Priority 5: Terraform and cloud path
 
 - [ ] Pass Terraform outputs into Ansible inventory generation.