Install the backend on every run, not on a checkout change

The role left musicdolphin with an empty venv and a service crash-looping on
ModuleNotFoundError while ansible reported failed=0, because installing the
backend hung off a handler notified by the git checkout:

  - the run where the checkout changed failed later on, so handlers never
    flushed and the notification was dropped;
  - the next run found the checkout already current, notified nothing, and
    installed nothing.

Nothing ever converges from there. Make it an ordinary task that runs every
time - uv is fast when there is nothing to do, and its output says whether it
actually installed anything, so a restart is still only notified on a real
change. Give the frontend the same treatment with a cheap stat, since the build
chain has the identical hole and a device with no dist serves no UI.

Also add libjpeg-dev and zlib1g-dev: Raspberry Pi OS has a 32-bit userland,
Pillow publishes no armv7 wheel, and uv therefore builds it from source.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

roles/pi_musicmouse/handlers/main.yml

@@ -1,10 +1,4 @@
 ---
-- name: Reinstall musicmouse backend
-  ansible.builtin.command:
-    cmd: "/usr/local/bin/uv pip install --python /opt/musicmouse/.venv/bin/python /opt/musicmouse/python-backend"
-  changed_when: true
-  notify: Restart musicmouse
-
 - name: Build frontend
   ansible.builtin.command:
     cmd: npm ci

roles/pi_musicmouse/tasks/main.yml

@@ -7,6 +7,11 @@
       - vlc-plugin-base
       - alsa-utils
       - samba
+      # Pillow ships no armv7 wheel and Raspberry Pi OS's userland is 32-bit, so it is
+      # compiled from source here and needs its image headers. (piwheels would have a
+      # prebuilt one, but only for the system Python, not the 3.13 uv installs.)
+      - libjpeg-dev
+      - zlib1g-dev
     cache_valid_time: 7200
 
 - name: Ensure uv install directory exists
@@ -46,7 +51,6 @@
     version: "{{ musicmouse_version }}"
     accept_hostkey: true
     force: true
-  notify: Reinstall musicmouse backend
 
 - name: Check the checkout has the layout this role knows
   ansible.builtin.stat:
@@ -73,6 +77,18 @@
     cmd: "/usr/local/bin/uv venv --python {{ pi_musicmouse_python_version }} /opt/musicmouse/.venv"
     creates: /opt/musicmouse/.venv/bin/python
 
+- name: Install the backend into the venv
+  # Every run, not on a checkout change via a handler. Tied to the checkout, one failed
+  # run strands the device: the notification is dropped when the play fails, the next
+  # run finds the checkout already current and notifies nothing, and the venv stays
+  # empty while the service crash-loops on ModuleNotFoundError. uv is fast when there
+  # is nothing to do, and its output says whether there was.
+  ansible.builtin.command:
+    cmd: "/usr/local/bin/uv pip install --python /opt/musicmouse/.venv/bin/python /opt/musicmouse/python-backend"
+  register: pi_musicmouse_install
+  changed_when: "'Installed' in pi_musicmouse_install.stderr"
+  notify: Restart musicmouse
+
 - name: Create media directory
   ansible.builtin.file:
     path: /media/musicmouse
@@ -141,6 +157,22 @@
     ansible_become: false  # see the first delegated task in tasks/main.yml
   notify: Build frontend
 
+- name: Check whether the device has a built frontend
+  ansible.builtin.stat:
+    path: /opt/musicmouse/web/dist/index.html
+  register: pi_musicmouse_dist
+
+- name: Queue a frontend build when the device has none
+  # The build chain hangs off the control machine's checkout changing, which has the
+  # same hole the backend install had: a run that fails after that checkout leaves the
+  # Pi with no dist, and every later run finds the checkout current and rebuilds
+  # nothing. Cheap to check, and it only fires when the device really has no UI.
+  ansible.builtin.debug:
+    msg: "No frontend at /opt/musicmouse/web/dist - queueing a build"
+  when: not pi_musicmouse_dist.stat.exists
+  changed_when: true
+  notify: Build frontend
+
 - name: Samba setup
   ansible.builtin.copy:
     src: smb.conf