Add Grafana Alloy and Loki for unified observability (#11)
## Summary
- Add Grafana Alloy to replace node_exporter for metrics collection
- Add Loki for log aggregation and storage
- Configure Alloy to collect logs from all services (grafana, forgejo, prometheus, tailscale, transmission, devpi, kiwix, borgmatic)
- Update Prometheus to accept metrics via remote_write
- Add Loki datasource to Grafana
## Test plan
- [ ] Run \`mise run provision-indri -- --check --diff\` to verify changes
- [ ] Apply with \`mise run provision-indri\`
- [ ] Verify services: \`mise run indri-services-check\`
- [ ] Check Grafana Explore with Loki datasource
- [ ] Query logs: \`{service="grafana"}\`
- [ ] Verify metrics still flowing to Prometheus dashboards
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Reviewed-on: https://forge.tail8d86e.ts.net/eblume/blumeops/pulls/11
Erich Blume
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