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_lifecycle_test.go

@@ -278,7 +278,7 @@ func TestEngine_StopHookHonoursStopTimeout(t *testing.T) {
 }
 
 // seedHeldDelivery gives the setup's webhook an http target with
-// maxRetries, pointed at a receiver that keeps every request open until
+// maxAttempts, pointed at a receiver that keeps every request open until
 // the client gives up on it, and seeds one delivery to that target at
 // status. It returns the task that sends attempt attemptNum of the
 // delivery, and a channel that is closed when a request reaches the
@@ -286,7 +286,7 @@ func TestEngine_StopHookHonoursStopTimeout(t *testing.T) {
 func seedHeldDelivery(
 	t *testing.T,
 	s iSetup,
-	maxRetries int,
+	maxAttempts int,
 	status database.DeliveryStatus,
 	attemptNum int,
 ) (delivery.Task, <-chan struct{}) {
@@ -315,7 +315,7 @@ func seedHeldDelivery(
 	iCreateWebhook(t, s.MainDB, s.WebhookID, "hook")
 	iCreateTarget(
 		t, s.MainDB, targetID, s.WebhookID, "held",
-		database.TargetTypeHTTP, cfg, maxRetries,
+		database.TargetTypeHTTP, cfg, maxAttempts,
 	)
 
 	event := iSeedEvent(t, s.WebhookDB, s.WebhookID, `{"held":true}`)
@@ -325,7 +325,7 @@ func seedHeldDelivery(
 
 	return iTask(
 		d, event, s.WebhookID, targetID, "held", cfg,
-		maxRetries, attemptNum, &bodyStr,
+		maxAttempts, attemptNum, &bodyStr,
 	), arrived
 }
 
@@ -380,7 +380,7 @@ func TestEngine_StopLeavesFireAndForgetAttemptPending(t *testing.T) {
 	lc := startEngineViaHook(t, s.Engine)
 
 	task, arrived := seedHeldDelivery(
-		t, s, 0, database.DeliveryStatusPending, 1,
+		t, s, 1, database.DeliveryStatusPending, 1,
 	)
 
 	s.Engine.Notify([]delivery.Task{task})
@@ -404,13 +404,13 @@ func TestEngine_StopLeavesFireAndForgetAttemptPending(t *testing.T) {
 func TestEngine_StopLeavesLastRetryAttemptRetrying(t *testing.T) {
 	t.Parallel()
 
-	const maxRetries = 2
+	const maxAttempts = 2
 
 	s := newISetup(t)
 	lc := startEngineViaHook(t, s.Engine)
 
 	task, arrived := seedHeldDelivery(
-		t, s, maxRetries, database.DeliveryStatusRetrying, maxRetries,
+		t, s, maxAttempts, database.DeliveryStatusRetrying, maxAttempts,
 	)
 	iSeedFailedResult(t, s.WebhookDB, task.DeliveryID)