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

An http target's max_queue_size was stored and shown in the target list as "Max Queue Size", but nothing in the delivery engine read it, so an operator who set it expecting deliveries to be bounded got nothing. It is removed from the target, the target list and the README's target table; neither form had a field for it. Nothing checks for a leftover value. 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.