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/playbooks/indri.yml

@@ -24,3 +24,5 @@
       tags: devpi
     - role: devpi_metrics
       tags: devpi_metrics
+    - role: plex_metrics
+      tags: plex_metrics