Add Plex Media Server observability (#13)

## Summary
- Add `plex_metrics` ansible role with textfile collector for Prometheus metrics
- Add Plex log collection to Alloy (forwards to Loki)
- Add Grafana dashboard for Plex monitoring (status, library counts, sessions, transcoding, logs)

## Metrics Collected
- `plex_up` - server health
- `plex_version_info` - server version
- `plex_sessions_total/playing/paused` - active sessions
- `plex_transcode_sessions_total/video/audio` - transcoding status
- `plex_library_items{library,type}` - library item counts

## Prerequisites
Plex token must be stored at `~/.plex-token` on indri (already done).

## Test plan
- [x] Dry-run passed (`mise run provision-indri -- --check --diff`)
- [ ] Apply changes (`mise run provision-indri`)
- [ ] Verify metrics: `ssh indri 'cat /opt/homebrew/var/node_exporter/textfile/plex.prom'`
- [ ] Verify logs in Grafana Explore: `{service="plex"}`
- [ ] Check Plex dashboard in Grafana

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: https://forge.tail8d86e.ts.net/eblume/blumeops/pulls/13

ansible/roles/plex_metrics/defaults/main.yml

@@ -0,0 +1,20 @@
+---
+# Plex metrics collection configuration
+
+# Plex server URL
+plex_url: "http://localhost:32400"
+
+# Path to file containing Plex token (should have 600 permissions)
+plex_token_file: "/Users/erichblume/.plex-token"
+
+# Metrics collection interval in seconds
+plex_metrics_interval: 60
+
+# Output directory for prometheus textfile collector
+plex_metrics_dir: /opt/homebrew/var/node_exporter/textfile
+
+# Script installation path
+plex_metrics_script: /Users/erichblume/bin/plex-metrics
+
+# Log directory for metrics script output
+plex_metrics_log_dir: /opt/homebrew/var/log