feat(monitoring): comprehensive alerting overhaul, 66 rules across 14 PrometheusRules

The Longhorn memory leak went undetected for 14 days because alerting
was broken (email receiver, missing label selector, no node alerts).
This overhaul brings alerting to production grade.

Fixes:
- Alloy Loki URL pointed to deleted loki-gateway, now loki:3100
- seaweedfs-bucket-init crash on unsupported `mc versioning` command
- All PrometheusRules now have `release: kube-prometheus-stack` label
- Removed broken email receiver, Matrix-only alerting

New alert coverage:
- Node: memory, CPU, swap, filesystem, inodes, network, clock skew, OOM
- Kubernetes: deployment down, CronJob failed, pod crash-looping, PVC full
- Backups: Postgres barman stale/failed, WAL archiving, SeaweedFS mirror
- Observability: Prometheus WAL/storage/rules, Loki/Tempo/AlertManager down
- Services: Stalwart, Bulwark, Tuwunel, Sol, Valkey, OpenSearch (smart)
- SLOs: auth stack 99.9% burn rate, Matrix 99.5%, latency p95 < 2s
- Recording rules for Linkerd RED metrics and node aggregates
- Watchdog heartbeat → Matrix every 12h (dead pipeline detection)
- Inhibition: critical suppresses warning for same alert+namespace
- OpenSearchClusterYellow only fires with >1 data node (single-node aware)

base/monitoring/alertrules-slo.yaml

@@ -0,0 +1,62 @@
+apiVersion: monitoring.coreos.com/v1
+kind: PrometheusRule
+metadata:
+  name: slo-alerts
+  namespace: monitoring
+  labels:
+    role: alert-rules
+    release: kube-prometheus-stack
+spec:
+  groups:
+    # SLO: Kratos/Hydra auth stack — 99.9% availability (43 min/month budget)
+    - name: slo-auth
+      rules:
+        - alert: AuthErrorBudgetFastBurn
+          expr: |
+            service:error_rate:5m{deployment=~"kratos|hydra"} > 0.0144
+          for: 2m
+          labels:
+            severity: critical
+            slo: auth-availability
+          annotations:
+            summary: "Auth stack burning error budget at 14.4x rate"
+            description: "{{ $labels.deployment }} error rate is {{ $value | humanizePercentage }} (14.4x burn rate for 99.9% SLO)."
+
+        - alert: AuthErrorBudgetSlowBurn
+          expr: |
+            service:error_rate:5m{deployment=~"kratos|hydra"} > 0.003
+          for: 1h
+          labels:
+            severity: warning
+            slo: auth-availability
+          annotations:
+            summary: "Auth stack slowly burning error budget"
+            description: "{{ $labels.deployment }} error rate is {{ $value | humanizePercentage }} (3x burn rate for 99.9% SLO)."
+
+    # SLO: Tuwunel Matrix homeserver — 99.5% availability (3.6 hr/month budget)
+    - name: slo-matrix
+      rules:
+        - alert: MatrixErrorBudgetFastBurn
+          expr: |
+            service:error_rate:5m{deployment="tuwunel"} > 0.072
+          for: 2m
+          labels:
+            severity: critical
+            slo: matrix-availability
+          annotations:
+            summary: "Matrix homeserver burning error budget at 14.4x rate"
+            description: "Tuwunel error rate is {{ $value | humanizePercentage }}."
+
+    # SLO: All services — latency p95 under 2s
+    - name: slo-latency
+      rules:
+        - alert: ServiceLatencyBudgetBurn
+          expr: |
+            service:latency_p95:5m > 2000
+          for: 10m
+          labels:
+            severity: warning
+            slo: latency
+          annotations:
+            summary: "Service p95 latency exceeds 2s SLO"
+            description: "{{ $labels.deployment }} in {{ $labels.namespace }} p95 latency is {{ $value }}ms."