Add bin/deploy:rollback to go back to a previous version

While I'm messing with production builds, I keep having to rollback a
bunch, so now we finally add tooling for it, lol

bin/deploy:rollback

@@ -0,0 +1,3 @@
+#!/usr/bin/env bash
+# Rollback to a previous version in production.
+cd $(dirname $0)/../deploy && ansible-playbook rollback.yml --extra-vars="new_app_version=$1"

deploy/rollback.yml

@@ -0,0 +1,71 @@
+---
+- name: Rollback impress to a previous version
+  hosts: webserver
+  become: yes
+  become_user: impress
+  vars:
+    remote_project_root: "/srv/impress"
+    ruby_version: "3.3.7"
+  tasks:
+    - name: Read the second-to-most-recent version
+      command:
+        chdir: "{{ remote_project_root }}/versions"
+        cmd: bash -c 'ls | tail -n 2 | head -n 1'
+      register: new_app_version
+
+    - name: Print out the new version name
+      debug:
+        msg: "Rolling back to version: {{ new_app_version.stdout }}"
+
+    - name: Save new remote folder path to a variable
+      set_fact:
+        remote_app_root: "{{ remote_project_root }}/versions/{{ new_app_version.stdout }}"
+
+    - name: Configure Bundler to run in deployment mode
+      command:
+        chdir: "{{ remote_app_root }}"
+        cmd: /opt/ruby-{{ ruby_version }}/bin/bundle config set --local deployment true
+
+    # This ensures that, while attempting our current deploy, we don't
+    # accidentally delete gems out from under the currently-running version.
+    # NOTE: From reading the docs, I thiink this is the default behavior, but
+    # I can't be sure? Rather than deep-dive to find out, I'd rather just set
+    # it, to be clear about the default(?) behavior we're depending on.
+    - name: Configure Bundler to *not* clean up old gems when installing
+      command:
+        chdir: "{{ remote_app_root }}"
+        cmd: /opt/ruby-{{ ruby_version }}/bin/bundle config set --local clean false
+
+    # NOTE: Bundler recommends this, and they're pretty smart about it: if the
+    # Gemfile changes, this shouldn't disrupt the currently-running version,
+    # because we won't clean up its now-unused gems yet, and if we upgrade a
+    # gem it'll install *both* versions of the gem until we clean up.
+    - name: Configure Bundler to use the bundle folder shared by all app versions
+      command:
+        chdir: "{{ remote_app_root }}"
+        cmd: "/opt/ruby-{{ ruby_version }}/bin/bundle config set --local path {{ remote_project_root}}/shared/bundle"
+
+    - name: Run `bundle install` to install dependencies in remote folder
+      command:
+        chdir: "{{ remote_app_root }}"
+        # The `--local` flag instructs Bundler to use the cached dependencies
+        # in `vendor/cache`, instead of reading from the web, which is much
+        # faster and more reliable!
+        cmd: /opt/ruby-{{ ruby_version }}/bin/bundle install --local
+
+    - name: Update the `current` folder to point to the new version
+      file:
+        src: "{{ remote_app_root }}"
+        dest: /srv/impress/current
+        state: link
+
+    # NOTE: This uses the passwordless sudo rule we set up in deploy:setup.
+    # We write it as a command rather than using the built-in `systemd` Ansible
+    # module, to make sure we're invoking it exactly as we wrote in that rule.
+    #
+    # NOTE: We use `sudo` instead of `become_user: root`, because we don't have
+    # permission to *become* the root user; we only have permission to run this
+    # one command as them.
+    - name: Restart the app
+      become: no
+      command: sudo systemctl restart impress