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/matrix/kustomization.yaml

@@ -13,3 +13,4 @@ resources:
   - hydra-oauth2client.yaml
   - sol-deployment.yaml
   - sol-config.yaml
+  - tuwunel-alertrules.yaml

base/matrix/tuwunel-alertrules.yaml

@@ -0,0 +1,31 @@
+apiVersion: monitoring.coreos.com/v1
+kind: PrometheusRule
+metadata:
+  name: tuwunel-alerts
+  namespace: matrix
+  labels:
+    role: alert-rules
+    release: kube-prometheus-stack
+spec:
+  groups:
+    - name: tuwunel
+      rules:
+        - alert: TuwunelDown
+          expr: |
+            kube_deployment_status_replicas_available{namespace="matrix", deployment="tuwunel"} == 0
+          for: 2m
+          labels:
+            severity: critical
+          annotations:
+            summary: "Matrix homeserver (Tuwunel) is down"
+            description: "Tuwunel is down — Matrix messaging, alertbot delivery, and Sol are all affected."
+
+        - alert: SolDown
+          expr: |
+            kube_deployment_status_replicas_available{namespace="matrix", deployment="sol"} == 0
+          for: 5m
+          labels:
+            severity: warning
+          annotations:
+            summary: "Sol is down"
+            description: "Sol virtual librarian is not running."