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_webhook.go

@@ -66,8 +66,9 @@ type Webhook struct {
 	// must equal DefaultRetentionDays.
 	RetentionDays int `gorm:"default:30" json:"retentionDays"`
 
-	// Relations
-	User        User         `json:"user,omitzero"`
+	// Relations. No model marshals the record it belongs to:
+	// User.Webhooks leads back here, and the JSON could loop.
+	User        User         `json:"-"`
 	Entrypoints []Entrypoint `json:"entrypoints,omitempty"`
 	Targets     []Target     `json:"targets,omitempty"`
 }