Replace useDashboardOverview, useDashboardPages, useDashboardLocations,
useDashboardDevices, useDashboardReferrers, useDashboardPerformance, and
useDashboardGoals with a single useDashboard hook that calls the existing
/dashboard batch endpoint. This endpoint runs all queries in parallel on
the backend and caches the result in Redis (30s TTL).
Reduces dashboard requests from 12 to 6 per refresh cycle (50% reduction).
At 1,000 concurrent users: ~6,000 req/min instead of 12,000.
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