grafana: prune stale dashboard files from prom on each apply

Without this, a renamed dashboard left the old file on disk and the
provisioner refused to write any dashboards because of a duplicate UID
(seen with towerops-phoenix-live-view vs towerops-phoenix_live_view).

ansible/roles/grafana/tasks/main.yml

@@ -69,6 +69,34 @@
     mode: "0644"
   notify: Restart grafana
 
+- name: List currently-provisioned dashboard files on host
+  ansible.builtin.find:
+    paths: "{{ grafana_dashboards_dir }}"
+    patterns: "*.json"
+    file_type: file
+  register: grafana_remote_dashboards
+
+- name: List dashboard JSON files from this role
+  ansible.builtin.set_fact:
+    grafana_managed_dashboards: >-
+      {{
+        query('ansible.builtin.fileglob',
+              role_path ~ '/files/dashboards/*.json')
+        | map('basename') | list
+      }}
+
+# Without this, a renamed dashboard leaves the old file on disk and the
+# provisioner refuses to write any dashboards (duplicate UID warning).
+- name: Remove stale dashboard files not present in the role
+  ansible.builtin.file:
+    path: "{{ item.path }}"
+    state: absent
+  loop: "{{ grafana_remote_dashboards.files }}"
+  loop_control:
+    label: "{{ item.path | basename }}"
+  when: (item.path | basename) not in grafana_managed_dashboards
+  notify: Restart grafana
+
 - name: Render dashboard provider config
   ansible.builtin.template:
     src: dashboard-providers.yaml.j2