feat(helm): Add observability support with ServiceMonitor and Grafana dashboard

Add comprehensive observability configuration to Helm chart:

**Helm Values:**
- Add observability configuration section for metrics, tracing, and logging
- Add serviceMonitor configuration (disabled by default)
- Add prometheusRule configuration (disabled by default)

**Templates:**
- Update deployment to include observability environment variables
- Update deployment to expose metrics port (9090)
- Update service to expose metrics port
- Add ServiceMonitor template for Prometheus Operator
- Add PrometheusRule template with critical and warning alerts

**Dashboards:**
- Add comprehensive Grafana dashboard JSON with 6 panels:
  - Request Rate (by method and endpoint)
  - Error Rate (5xx errors percentage)
  - Request Latency (P50/P95 by endpoint)
  - Top MCP Tools (by invocation volume)
  - Nextcloud API Latency (by app)
  - Vector Sync Queue Size
- Add dashboard README with import instructions

**Alert Rules:**
- Critical: Server down, high error rate (>5%), high latency (>1s), dependency down
- Warning: Token validation errors (>1%), vector sync queue high (>100), Qdrant slow (>500ms)

All features are opt-in via values.yaml configuration.

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

Co-Authored-By: Claude <noreply@anthropic.com>

charts/nextcloud-mcp-server/templates/servicemonitor.yaml

@@ -0,0 +1,32 @@
+{{- if and .Values.observability.metrics.enabled .Values.serviceMonitor.enabled }}
+apiVersion: monitoring.coreos.com/v1
+kind: ServiceMonitor
+metadata:
+  name: {{ include "nextcloud-mcp-server.fullname" . }}
+  namespace: {{ .Release.Namespace }}
+  labels:
+    {{- include "nextcloud-mcp-server.labels" . | nindent 4 }}
+    {{- with .Values.serviceMonitor.labels }}
+    {{- toYaml . | nindent 4 }}
+    {{- end }}
+spec:
+  selector:
+    matchLabels:
+      {{- include "nextcloud-mcp-server.selectorLabels" . | nindent 6 }}
+  endpoints:
+    - port: metrics
+      path: {{ .Values.observability.metrics.path }}
+      interval: {{ .Values.serviceMonitor.interval }}
+      scrapeTimeout: {{ .Values.serviceMonitor.scrapeTimeout }}
+      scheme: http
+      relabelings:
+        # Add namespace label
+        - sourceLabels: [__meta_kubernetes_namespace]
+          targetLabel: namespace
+        # Add pod label
+        - sourceLabels: [__meta_kubernetes_pod_name]
+          targetLabel: pod
+        # Add service label
+        - sourceLabels: [__meta_kubernetes_service_name]
+          targetLabel: service
+{{- end }}