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/values.yaml

@@ -168,6 +168,43 @@ securityContext:
   runAsNonRoot: true
   runAsUser: 1000
 
+# Observability Configuration
+observability:
+  # Prometheus metrics
+  metrics:
+    enabled: true
+    port: 9090
+    path: /metrics
+
+  # OpenTelemetry tracing
+  tracing:
+    enabled: false
+    endpoint: ""  # e.g., "http://opentelemetry-collector:4317"
+    serviceName: "nextcloud-mcp-server"
+    samplingRate: 1.0
+
+  # Logging configuration
+  logging:
+    format: json  # "json" or "text"
+    level: INFO
+    includeTraceContext: true
+
+# Prometheus ServiceMonitor (requires Prometheus Operator)
+serviceMonitor:
+  enabled: false
+  interval: 30s
+  scrapeTimeout: 10s
+  labels: {}
+  # Additional labels for ServiceMonitor (e.g., for Prometheus selector)
+  # Example: { prometheus: kube-prometheus }
+
+# Prometheus alert rules (requires Prometheus Operator)
+prometheusRule:
+  enabled: false
+  labels: {}
+  # Additional labels for PrometheusRule (e.g., for Prometheus selector)
+  # Example: { prometheus: kube-prometheus }
+
 service:
   type: ClusterIP
   port: 8000