Keep each model's parent out of its JSON (closes #177)

Every reference from a model to the record it belongs to is now
json:"-", so the webhook and target models (and the other parent and
child pairs in internal/database) can no longer marshal each other in
a loop. Preloading maps associations without reading json tags, so it
still fills these references; a new test preloads a webhook with its targets and
entrypoints and a target with its webhook, checks the references are
filled, and checks the marshalled JSON leaves them out.

Model: opus-5-5

internal/database/model_delivery.go

@@ -56,8 +56,10 @@ type Delivery struct {
 	// the index.
 	FinishedAt *time.Time `gorm:"index:idx_deliveries_status,priority:3" json:"finishedAt,omitempty"`
 
-	// Relations
-	Event           Event            `json:"event,omitzero"`
-	Target          Target           `json:"target,omitzero"`
+	// Relations. No model marshals the record it belongs to:
+	// Event.Deliveries and Target.Deliveries lead back here, and the
+	// JSON could loop.
+	Event           Event            `json:"-"`
+	Target          Target           `json:"-"`
 	DeliveryResults []DeliveryResult `json:"deliveryResults,omitempty"`
 }