docs(setup): make fail2ban opt-in in 70-hardening.sh (#222)

## Summary

Follow-up on [#220](#220) / [#221](#221). Makes fail2ban **opt-in** in [`70-hardening.sh`](docs/setup/scripts/70-hardening.sh) instead of installing it unconditionally.

Reasoning: some corp environments already ship brute-force protection at the network layer (ACL / corp firewall / appliance) — fail2ban on the host then becomes redundant and can be the wrong layer to debug from when a rule misfires. The other three hardening steps (UFW enable, sshd lockdown) were already prompt-gated; fail2ban was the odd one out.

## What lands

| File | Change |
| --- | --- |
| `docs/setup/scripts/70-hardening.sh` | fail2ban block restructured into three branches: (1) already running → skip; (2) installed but stopped → prompt to enable+start; (3) not installed → prompt to install+enable+start. Each "no" path logs `↪ skip (user choice)` so re-runs don't repeatedly nag if the dev has already declined. |
| `docs/setup/01-dev-debian-vm-setup.md` | Table row for `70-hardening.sh` clarified — each sub-step's prompt posture is now visible: UFW prompts before enabling, fail2ban prompts before installing, sshd hardening prompts before applying. `unattended-upgrades` is the only one applied unconditionally. |
| `docs/setup/README.md` | Same descriptor adjustment. |

## Test plan

- [ ] On a fresh Debian VM with no fail2ban installed, run `70-hardening.sh`, decline the fail2ban prompt → script continues, fail2ban not installed, no service started.
- [ ] On the same VM, re-run `70-hardening.sh` → the fail2ban branch prompts again (the dev may have changed their mind); declining again produces the same `↪ skip (user choice)` result.
- [ ] On a VM where fail2ban is pre-installed but stopped (rare, but possible if infra rolled it back), the script offers to start it without re-installing.
- [x] `pnpm exec prettier --check` clean on the touched files.
- [x] `bash -n docs/setup/scripts/70-hardening.sh` (syntax check) clean.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #222

docs/setup/README.md

@@ -23,7 +23,7 @@ Modular setup scripts called from the doc above. Each is idempotent, each can be
 | [`40-node.sh`](scripts/40-node.sh)                   | nvm + Node from the repo's `.nvmrc` + corepack pinned to `package.json#packageManager`'s pnpm version.                                                                           |
 | [`50-docker.sh`](scripts/50-docker.sh)               | Docker CE + compose plugin from docker.com apt repo + user added to `docker` group + service enabled at boot.                                                                    |
 | [`60-tuning.sh`](scripts/60-tuning.sh)               | inotify watches → 524288, optional 4 GB swapfile, optional hostname rename (only prompts if generic).                                                                            |
-| [`70-hardening.sh`](scripts/70-hardening.sh)         | Best-effort UFW + unattended-upgrades + fail2ban + sshd hardening drop-in. Probes before applying — skips done items.                                                            |
+| [`70-hardening.sh`](scripts/70-hardening.sh)         | Best-effort UFW + unattended-upgrades + fail2ban (opt-in) + sshd hardening drop-in. Probes before applying — skips done items, prompts before each install.                      |
 | [`80-dotfiles.sh`](scripts/80-dotfiles.sh)           | Symlinks `notes/aliases.zsh` → `~/.oh-my-zsh/custom/aliases.zsh`. Copies `notes/gitconfig.txt` to `~/.gitconfig` with prompted identity.                                         |
 
 ## `systemd/`