[Studio] Derive dashboard summary from clusters

[Aias00]

Summary

• replace the hard-coded dashboard provider with a repository-backed provider
• summarize cluster, broker, proxy, nameserver, topic, group, and TPS totals from ClusterRepository
• avoid fabricated daily message totals until real Prometheus-backed metrics are wired

Scope

• Track 1 / BASE-01 dashboard baseline
• Supports METRICS-01 by keeping the dashboard backend aligned with the cluster model before real Prometheus dashboards are added

Validation

• JAVA_HOME=/Users/aias/Library/Java/JavaVirtualMachines/openjdk-21.0.2/Contents/Home mvn -Dtest=DashboardServiceTest,DashboardControllerTest,RepositoryDashboardProviderTest test
• JAVA_HOME=/Users/aias/Library/Java/JavaVirtualMachines/openjdk-21.0.2/Contents/Home mvn test
• git diff --check

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[Aias00]

Superseded by #642. Folded into the consolidated AI resource tools PR because the dashboard summary tool consumes DashboardService/DashboardProvider, so repository-backed dashboard summaries belong with the AI dashboard tool review.