feat(dashboard): real business metrics with week-over-week trends

Dashboard KPIs were three flat counters with no trend data — the cards
rendered a neutral dash where the design intends movement indicators.
Summary now queries three trailing windows and emits four KPIs with
real deltas: bookings this week, completed (new CompletedCount metric
in both repositories), cancellations, and utilization in percentage
points. KPI labels localize in the frontend via a code-to-key map so
Czech operators no longer see English labels on live data.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/openapi/bookra.openapi.yaml

@@ -1624,6 +1624,11 @@ components:
           type: string
         value:
           type: string
+        change:
+          type: string
+        trend:
+          type: string
+          enum: [up, down, neutral]
     DashboardSummary:
       type: object
       required: [tenantName, tenantSlug, locale, timezone, planCode, publicBookingUrl, setupCompletion, kpis]