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_target.go

@@ -34,7 +34,8 @@ type Target struct {
 	MaxRetries   int `json:"maxRetries,omitempty"`
 	MaxQueueSize int `json:"maxQueueSize,omitempty"`
 
-	// Relations
-	Webhook    Webhook    `json:"webhook,omitzero"`
+	// Relations. No model marshals the record it belongs to:
+	// Webhook.Targets leads back here, and the JSON could loop.
+	Webhook    Webhook    `json:"-"`
 	Deliveries []Delivery `json:"deliveries,omitempty"`
 }