Remove an http target's max_queue_size (closes #477)

Nothing in delivery ever read it, so setting it bounded nothing. The
field, the "Max Queue Size" line in the target list, its README row and
the tests that set or checked it are gone. An existing database keeps
its old column, which is no longer read.

Model: opus-5-5

internal/database/model_target.go

@@ -31,8 +31,7 @@ type Target struct {
 
 	// For HTTP targets (max_retries=0 means fire-and-forget,
 	// >0 enables retries with backoff)
-	MaxRetries   int `json:"maxRetries,omitempty"`
-	MaxQueueSize int `json:"maxQueueSize,omitempty"`
+	MaxRetries int `json:"maxRetries,omitempty"`
 
 	// Relations. No model marshals the record it belongs to:
 	// Webhook.Targets leads back here, and the JSON could loop.