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/transmission_metrics/templates/transmission-metrics.plist.j2

@@ -0,0 +1,26 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- {{ ansible_managed }} -->
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>Label</key>
+	<string>mcquack.eblume.transmission-metrics</string>
+	<key>EnvironmentVariables</key>
+	<dict>
+		<key>PATH</key>
+		<string>/opt/homebrew/bin:/usr/bin:/bin</string>
+	</dict>
+	<key>ProgramArguments</key>
+	<array>
+		<string>{{ transmission_metrics_script }}</string>
+	</array>
+	<key>StartInterval</key>
+	<integer>{{ transmission_metrics_interval }}</integer>
+	<key>RunAtLoad</key>
+	<true/>
+	<key>StandardErrorPath</key>
+	<string>{{ transmission_log_dir }}/transmission-metrics.err.log</string>
+	<key>StandardOutPath</key>
+	<string>{{ transmission_log_dir }}/transmission-metrics.out.log</string>
+</dict>
+</plist>