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/target_http.go

@@ -53,10 +53,10 @@ type httpCore struct {
 }
 
 // deliver runs one delivery attempt through the retry core.
-// A maxRetries of 0 is fire-and-forget: a single attempt is
-// recorded and no circuit breaker is consulted. A positive
-// maxRetries gates the attempt on the circuit breaker and
-// schedules a backed-off retry on failure.
+// A maxAttempts of 1 is a single attempt, recorded without
+// consulting a circuit breaker. A higher maxAttempts gates the
+// attempt on the circuit breaker and schedules a backed-off retry
+// on failure, until maxAttempts attempts have been made.
 //
 // ctx is the context attempt sends its request with. Stopping the
 // engine cancels it; see interruptedByStop.
@@ -66,17 +66,17 @@ func (c *httpCore) deliver(
 	d *database.Delivery,
 	task *Task,
 	sched Scheduler,
-	maxRetries int,
+	maxAttempts int,
 	attempt func() attemptResult,
 ) {
-	if maxRetries == 0 {
+	if maxAttempts <= 1 {
 		c.fireAndForget(ctx, webhookDB, d, attempt())
 
 		return
 	}
 
 	c.withRetry(
-		ctx, webhookDB, d, task, sched, maxRetries, attempt,
+		ctx, webhookDB, d, task, sched, maxAttempts, attempt,
 	)
 }
 
@@ -153,7 +153,7 @@ func (c *httpCore) withRetry(
 	d *database.Delivery,
 	task *Task,
 	sched Scheduler,
-	maxRetries int,
+	maxAttempts int,
 	attempt func() attemptResult,
 ) {
 	cb := c.getCircuitBreaker(task.TargetID)
@@ -205,7 +205,7 @@ func (c *httpCore) withRetry(
 	cb.RecordFailure()
 
 	c.handleRetry(
-		webhookDB, d, task, sched, maxRetries, attemptNum,
+		webhookDB, d, task, sched, maxAttempts, attemptNum,
 	)
 }
 
@@ -266,10 +266,10 @@ func (c *httpCore) handleRetry(
 	d *database.Delivery,
 	task *Task,
 	sched Scheduler,
-	maxRetries int,
+	maxAttempts int,
 	attemptNum int,
 ) {
-	if attemptNum >= maxRetries {
+	if attemptNum >= maxAttempts {
 		c.eng.settleStatus(
 			webhookDB, d, d.Target.Type,
 			database.DeliveryStatusFailed,
@@ -439,7 +439,7 @@ func (t *httpTarget) Deliver(
 
 	t.deliver(
 		ctx, webhookDB, d, task, sched,
-		d.Target.MaxRetries, attempt,
+		d.Target.MaxAttempts, attempt,
 	)
 }