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

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

Reviewed-on: https://forge.tail8d86e.ts.net/eblume/blumeops/pulls/11

ansible/playbooks/indri.yml

@@ -2,8 +2,10 @@
 - name: Configure indri
   hosts: indri
   roles:
-    - role: node_exporter
-      tags: node_exporter
+    - role: loki
+      tags: loki
+    - role: alloy
+      tags: alloy
     - role: prometheus
       tags: prometheus
     - role: grafana