feat: add JSON serialization, urgency field, and snake_case API contract

Fix latent API bug where multi-word fields (RecurrenceDuration, ParentUUID,
CreatedAt) serialized as PascalCase, breaking the frontend. Add explicit
snake_case json tags and custom MarshalJSON/UnmarshalJSON on Task, Status,
and APIKey to emit unix timestamps and string status codes.

Add Urgency float64 as a derived field on Task, populated via
PopulateUrgency helper in all handlers before serialization. The report
engine's sortByUrgency now also retains the computed score.

Frontend updated with urgency type, color-coded badge in TaskItem, and
mock data values.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

opal-task/internal/engine/report.go

@@ -420,7 +420,8 @@ func mergeFilters(base, user *Filter) *Filter {
 	return merged
 }
 
-// sortByUrgency is a helper function to sort tasks by urgency (descending)
+// sortByUrgency is a helper function to sort tasks by urgency (descending).
+// It also populates the Urgency field on each task so the score is available in responses.
 func sortByUrgency(tasks []*Task) []*Task {
 	cfg, _ := GetConfig()
 	coeffs := BuildUrgencyCoefficients(cfg)
@@ -428,11 +429,14 @@ func sortByUrgency(tasks []*Task) []*Task {
 	sorted := make([]*Task, len(tasks))
 	copy(sorted, tasks)
 
+	// Calculate and store urgency on each task
+	for _, t := range sorted {
+		t.Urgency = t.CalculateUrgency(coeffs)
+	}
+
 	for i := 0; i < len(sorted)-1; i++ {
 		for j := i + 1; j < len(sorted); j++ {
-			urgI := sorted[i].CalculateUrgency(coeffs)
-			urgJ := sorted[j].CalculateUrgency(coeffs)
-			if urgI < urgJ {
+			if sorted[i].Urgency < sorted[j].Urgency {
 				sorted[i], sorted[j] = sorted[j], sorted[i]
 			}
 		}