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; 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/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,