feat: add dashboard aggregation endpoint (Phase 2I)

GET /api/dashboard returns aggregated data:
- deadline_summary: overdue, due this/next week, ok counts
- case_summary: active, new this month, closed counts
- upcoming_deadlines: next 7 days with case info
- upcoming_appointments: next 7 days
- recent_activity: last 10 case events

Uses efficient CTE query for summaries. Also fixes duplicate
writeJSON/writeError declarations in appointments handler.

backend/internal/services/dashboard_service_test.go

@@ -0,0 +1,33 @@
+package services
+
+import (
+	"testing"
+	"time"
+)
+
+func TestDashboardDateCalculations(t *testing.T) {
+	// Verify the date range logic used in Get()
+	now := time.Date(2026, 3, 25, 14, 0, 0, 0, time.UTC) // Wednesday
+
+	today := now.Format("2006-01-02")
+	endOfWeek := now.AddDate(0, 0, 7-int(now.Weekday())).Format("2006-01-02")
+	endOfNextWeek := now.AddDate(0, 0, 14-int(now.Weekday())).Format("2006-01-02")
+	in7Days := now.AddDate(0, 0, 7).Format("2006-01-02")
+	startOfMonth := time.Date(now.Year(), now.Month(), 1, 0, 0, 0, 0, now.Location()).Format("2006-01-02")
+
+	if today != "2026-03-25" {
+		t.Errorf("today = %s, want 2026-03-25", today)
+	}
+	if endOfWeek != "2026-03-29" { // Sunday
+		t.Errorf("endOfWeek = %s, want 2026-03-29", endOfWeek)
+	}
+	if endOfNextWeek != "2026-04-05" {
+		t.Errorf("endOfNextWeek = %s, want 2026-04-05", endOfNextWeek)
+	}
+	if in7Days != "2026-04-01" {
+		t.Errorf("in7Days = %s, want 2026-04-01", in7Days)
+	}
+	if startOfMonth != "2026-03-01" {
+		t.Errorf("startOfMonth = %s, want 2026-03-01", startOfMonth)
+	}
+}