Merge pull request 'Add initial project structure and documentation' (#6) from chore/documentation into main

Reviewed-on: #6

roles/upgrade/README.md

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+# Upgrade role
+
+Upgrades all packages using the native package manager (`apk` or `apt`).
+
+## Problem
+
+Tailscale may be updated during the upgrade. When its service restarts, the SSH tunnel drops and Ansible loses connectivity, causing a fatal `UNREACHABLE`.
+
+## Solution — async + poll
+
+Instead of a blocking upgrade, we fire the job in the background and poll for completion, tolerating temporary unreachability.
+
+```mermaid
+sequenceDiagram
+    participant A as Ansible
+    participant T as Target
+
+    A->>T: apk upgrade (async, poll: 0)
+    T-->>A: job ID: 42
+    Note over T: 🔄 upgrade en cours...
+    A->>T: async_status jid=42
+    T-->>A: finished: false
+    Note over A: FAILED-RETRYING (normal)
+    A-->>A: sleep 10s
+    A->>T: async_status jid=42
+    T-->>A: finished: false
+    Note over A: FAILED-RETRYING (normal)
+    A-->>A: sleep 10s
+    A->>T: async_status jid=42
+    T-->>A: finished: true ✅
+    Note over A: ok
+```
+
+- **`async: 3600` + `poll: 0`** — fire and forget. Ansible gets a job ID and moves on.
+- **`async_status` with `until: finished`** — polls every 10s for up to 60 retries (~10 min).
+- **`ignore_unreachable: true`** — if Tailscale dropped SSH, Ansible doesn't crash. It retries until the host is back.
+- The `FAILED - RETRYING` messages are normal — they just mean the `until` condition isn't met yet.
+
+## Idempotency
+
+A task with `async` + `poll: 0` always reports `changed: true` by default, even when nothing happened. To fix this:
+
+```mermaid
+graph LR
+    A["Fire: changed_when: false"] --> B["Package manager runs in background"]
+    B --> C["Wait: polls every 10s"]
+    C -->|finished| D["async_status reads real changed value"]
+    D -->|"packages updated"| E["changed ✅"]
+    D -->|"nothing to do"| F["ok"]
+```
+
+- The **fire** task has `changed_when: false` — it never claims change.
+- The **wait** task reads `changed_when: async_result.changed` — it propagates the real status from the package manager once the job finishes.
+
+Second run with nothing to upgrade → `changed=0` on all hosts.