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_event.go

@@ -44,8 +44,9 @@ type Event struct {
 	// kept as the record of where the copy came from either way.
 	ResubmittedFromID *string `gorm:"type:uuid;index" json:"resubmittedFromId,omitempty"`
 
-	// Relations
-	Webhook    Webhook    `json:"webhook,omitzero"`
-	Entrypoint Entrypoint `json:"entrypoint,omitzero"`
+	// Relations. No model marshals the record it belongs to, so
+	// Webhook and Entrypoint are left out of the JSON.
+	Webhook    Webhook    `json:"-"`
+	Entrypoint Entrypoint `json:"-"`
 	Deliveries []Delivery `json:"deliveries,omitempty"`
 }