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_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)