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. The column is NOT NULL and a check constraint refuses a value
below 1, so every target, whatever its type, is stored with at least
1. 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; a blank field on the add target form gives 1, and on the
edit form keeps the stored value. 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 +29,12 @@ 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. The table's check constraint refuses
+	// a target saved with less than 1, whatever its type.
+	MaxAttempts int `gorm:"not null;check:max_attempts >= 1" json:"maxAttempts,omitempty"`
 
 	// Relations. No model marshals the record it belongs to:
 	// Webhook.Targets leads back here, and the JSON could loop.