Add OpenTelemetry distributed tracing (Tempo + Beyla eBPF) (#286)

## Summary

Adds the third observability pillar — **distributed tracing** — alongside existing metrics (Prometheus) and logs (Loki).

- **Grafana Tempo 2.10.1** on minikube-indri for trace storage with 7d retention, OTLP receivers, and `metrics_generator` that remote-writes span-metrics (RED) to Prometheus
- **Beyla eBPF auto-instrumentation** via a privileged Alloy DaemonSet on ringtail — instruments HTTP services (Frigate, ntfy, Ollama, Immich) without code changes
- **Grafana integration** — Tempo datasource with trace↔log and trace↔metrics correlation, plus Loki derivedFields for trace ID linking
- **Prometheus** scrapes Tempo operational metrics

### Architecture

```
ringtail (k3s)                                indri (minikube)
┌──────────────────────┐                      ┌─────────────────────┐
│ Alloy+Beyla (eBPF)   │──OTLP HTTP────────→ │ Tempo               │
│  ↳ Frigate, ntfy,    │  via tailnet         │  ↳ trace storage    │
│    Ollama, Immich     │                      │  ↳ RED → Prometheus │
└──────────────────────┘                      │                     │
                                              │ Grafana             │
                                              │  ↳ Tempo datasource │
                                              └─────────────────────┘
```

### New files (12)
- `docs/reference/services/tempo.md` — reference doc
- `docs/changelog.d/feature-otel-tracing.feature.md`
- `argocd/apps/tempo.yaml` + `argocd/manifests/tempo/` (6 files)
- `argocd/apps/alloy-tracing-ringtail.yaml` + `argocd/manifests/alloy-tracing-ringtail/` (4 files)

### Modified files (6)
- `argocd/manifests/grafana/datasources.yaml` — Tempo datasource + Loki derivedFields
- `argocd/manifests/prometheus/prometheus.yml` — Tempo scrape target
- `service-versions.yaml` — tempo + alloy-tracing-ringtail entries
- `docs/reference/services/grafana.md` — Tempo in datasources table
- `docs/reference/reference.md` — Tempo in services index
- `docs/reference/operations/observability.md` — Tempo in components list

## Deployment and Testing

- [ ] Sync `apps` app to pick up new Application definitions
- [ ] `argocd app set tempo --revision feature/otel-tracing && argocd app sync tempo`
- [ ] Verify Tempo pod: `kubectl --context=minikube-indri get pods -n monitoring -l app=tempo`
- [ ] Verify Tempo ready: port-forward 3200 and `curl localhost:3200/ready`
- [ ] Verify Tailscale ingresses: `kubectl --context=minikube-indri get ingress -n monitoring`
- [ ] `argocd app set alloy-tracing-ringtail --revision feature/otel-tracing && argocd app sync alloy-tracing-ringtail`
- [ ] Check Beyla discovery in alloy-tracing logs on ringtail
- [ ] Sync grafana-config for updated datasources
- [ ] Sync prometheus for updated scrape config
- [ ] Test Grafana Tempo datasource connection
- [ ] Generate test traffic and search traces in Grafana Explore → Tempo
- [ ] After merge: reset all ArgoCD app revisions back to main

Reviewed-on: #286

docs/reference/kubernetes/apps.md

@@ -27,7 +27,9 @@ Registry of all applications deployed via [[argocd]].
 | `grafana` | monitoring | Helm chart (forge mirror) | [[grafana]] |
 | `grafana-config` | monitoring | `argocd/manifests/grafana-config/` | [[grafana]] |
 | `immich` | immich | Helm chart | [[immich]] |
+| `tempo` | monitoring | `argocd/manifests/tempo/` | [[tempo]] |
 | `alloy-k8s` | alloy | `argocd/manifests/alloy-k8s/` | [[alloy|Alloy]] |
+| `alloy-tracing-ringtail` | alloy | `argocd/manifests/alloy-tracing-ringtail/` | [[alloy|Alloy]] (eBPF tracing) |
 | `kube-state-metrics` | monitoring | `argocd/manifests/kube-state-metrics/` | K8s metrics |
 | `miniflux` | miniflux | `argocd/manifests/miniflux/` | [[miniflux]] |
 | `kiwix` | kiwix | `argocd/manifests/kiwix/` | [[kiwix]] |