Store a target's number of delivery attempts, at least one (closes #540)

Target.MaxRetries (max_retries) becomes Target.MaxAttempts
(max_attempts), and the form field, parser and ceiling are renamed to
match. 1 is a single attempt with no circuit breaker; above 1 retries
behind the breaker up to that many attempts. Both target forms refuse
0 and less and default a blank field to 1. The target page shows the
plain number, and the README's two paragraphs on 0 against 1 become
one sentence.

Model: opus-5-5

internal/database/model_target.go

@@ -29,9 +29,11 @@ type Target struct {
 	// handler that marshals the model itself.
 	Config string `gorm:"type:text" json:"-"` // JSON configuration
 
-	// For HTTP targets (max_retries=0 means fire-and-forget,
-	// >0 enables retries with backoff)
-	MaxRetries int `json:"maxRetries,omitempty"`
+	// MaxAttempts is how many times an HTTP or Slack target attempts
+	// each delivery in all, at least 1. 1 is a single attempt with no
+	// circuit breaker; above 1, a failed attempt is retried with
+	// backoff behind the breaker.
+	MaxAttempts int `json:"maxAttempts,omitempty"`
 
 	// Relations. No model marshals the record it belongs to:
 	// Webhook.Targets leads back here, and the JSON could loop.