fix: eliminate N+1 dashboard metrics API requests

[Arryan21-06]

Summary

Closes #2395

What Changed

• Centralized dashboard metrics fetching.
• Added a shared DashboardMetricsContext for dashboard widgets.
• Replaced multiple independent client-side metric requests with a single aggregated metrics request.
• Added a consolidated /api/metrics endpoint.
• Updated dashboard components to consume shared metrics instead of fetching independently.
• Reduced redundant API calls and improved dashboard loading performance.

Benefits

• Eliminates the N+1 API request pattern.
• Reduces GitHub API usage and rate-limit risk.
• Provides a more consistent loading experience.
• Improves dashboard scalability and maintainability.

Testing

• Dashboard loads successfully.
• Verified dashboard metrics render correctly.
• Confirmed consolidated metrics fetching.
• Existing dashboard functionality remains intact.

[github-actions]

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