Add transmission dashboard to grafana

- Add node_exporter ansible role to enable textfile collector
- Add transmission_metrics role with script and LaunchAgent
  - Collects metrics every 60s via transmission RPC
  - Writes to /opt/homebrew/var/node_exporter/textfile/transmission.prom
- Update grafana role to provision dashboards from files
- Add transmission.json dashboard with:
  - Status indicator, torrent counts
  - Transfer speeds, cumulative stats
  - Time series graphs

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

ansible/roles/node_exporter/tasks/main.yml

@@ -0,0 +1,22 @@
+---
+# Note: node_exporter is installed via homebrew manually.
+# This role manages the args file to enable textfile collector.
+
+- name: Create textfile collector directory
+  ansible.builtin.file:
+    path: "{{ node_exporter_textfile_dir }}"
+    state: directory
+    mode: '0755'
+
+- name: Configure node_exporter args
+  ansible.builtin.template:
+    src: node_exporter.args.j2
+    dest: /opt/homebrew/etc/node_exporter.args
+    mode: '0644'
+  notify: restart node_exporter
+
+- name: Ensure node_exporter service is started
+  ansible.builtin.command: brew services start node_exporter
+  register: brew_start
+  changed_when: "'Successfully started' in brew_start.stdout"
+  failed_when: false