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/internal/domain/models.go

@@ -10,9 +10,11 @@ type Principal struct {
 }
 
 type DashboardKPI struct {
-	Code  string `json:"code"`
-	Label string `json:"label"`
-	Value string `json:"value"`
+	Code   string `json:"code"`
+	Label  string `json:"label"`
+	Value  string `json:"value"`
+	Change string `json:"change,omitempty"`
+	Trend  string `json:"trend,omitempty"`
 }
 
 type UpcomingBooking struct {