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/delivery/engine.go

@@ -107,7 +107,7 @@ type Task struct {
 	TargetName   string
 	TargetType   database.TargetType
 	TargetConfig string
-	MaxRetries   int
+	MaxAttempts  int
 
 	Method      string
 	RawQuery    string
@@ -1769,10 +1769,10 @@ func buildEventFromTask(task *Task) database.Event {
 
 func buildTargetFromTask(task *Task) database.Target {
 	target := database.Target{
-		Name:       task.TargetName,
-		Type:       task.TargetType,
-		Config:     task.TargetConfig,
-		MaxRetries: task.MaxRetries,
+		Name:        task.TargetName,
+		Type:        task.TargetType,
+		Config:      task.TargetConfig,
+		MaxAttempts: task.MaxAttempts,
 	}
 
 	target.ID = task.TargetID
@@ -2028,7 +2028,7 @@ func (e *Engine) countAttemptsBatch(
 	// One delivery id per recorded attempt, tallied here rather than
 	// grouped in SQL: internal/gormlog forbids (*gorm.DB).Scan, which
 	// a GROUP BY into a struct would need, and an attempt row per
-	// delivery is bounded by the target's MaxRetries.
+	// delivery is bounded by the target's MaxAttempts.
 	var attemptIDs []string
 
 	err := webhookDB.
@@ -2106,7 +2106,7 @@ func buildRecoveryTask(
 		TargetName:   target.Name,
 		TargetType:   target.Type,
 		TargetConfig: target.Config,
-		MaxRetries:   target.MaxRetries,
+		MaxAttempts:  target.MaxAttempts,
 		Method:       event.Method,
 		RawQuery:     event.RawQuery,
 		Headers:      event.Headers,