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/metrics_test.go

@@ -291,20 +291,20 @@ func TestDeliveryMetrics_CircuitBreakerGauge(t *testing.T) {
 	body := event.Body
 	cfg := iHTTPConfig(ts.URL)
 
-	// A retry budget above the failure threshold, so the breaker
+	// An attempt budget above the failure threshold, so the breaker
 	// rather than the budget is what stops the delivery.
-	maxRetries := delivery.ExportDefaultFailureThreshold + 5
+	maxAttempts := delivery.ExportDefaultFailureThreshold + 5
 
 	// The retries below are only run if their target still exists;
 	// see https://git.eeqj.de/sneak/webhooker/issues/107.
 	iCreateTarget(
 		t, s.MainDB, targetID, s.WebhookID, "metrics-trip",
-		database.TargetTypeHTTP, cfg, maxRetries,
+		database.TargetTypeHTTP, cfg, maxAttempts,
 	)
 
 	first := iTask(
 		d, event, s.WebhookID, targetID,
-		"metrics-trip", cfg, maxRetries, 1, &body,
+		"metrics-trip", cfg, maxAttempts, 1, &body,
 	)
 
 	s.Engine.ExportProcessNewTask(context.TODO(), &first)
@@ -316,7 +316,7 @@ func TestDeliveryMetrics_CircuitBreakerGauge(t *testing.T) {
 		ExportDefaultFailureThreshold; attempt++ {
 		task := iTask(
 			d, event, s.WebhookID, targetID,
-			"metrics-trip", cfg, maxRetries, attempt, &body,
+			"metrics-trip", cfg, maxAttempts, attempt, &body,
 		)
 
 		s.Engine.ExportProcessRetryTask(
@@ -364,16 +364,16 @@ func TestDeliveryMetrics_BreakerBlockedIsNotAnAttempt(
 
 	body := event.Body
 	cfg := iHTTPConfig(ts.URL)
-	maxRetries := delivery.ExportDefaultFailureThreshold + 5
+	maxAttempts := delivery.ExportDefaultFailureThreshold + 5
 
 	iCreateTarget(
 		t, s.MainDB, targetID, s.WebhookID, "metrics-blocked",
-		database.TargetTypeHTTP, cfg, maxRetries,
+		database.TargetTypeHTTP, cfg, maxAttempts,
 	)
 
 	first := iTask(
 		d, event, s.WebhookID, targetID,
-		"metrics-blocked", cfg, maxRetries, 1, &body,
+		"metrics-blocked", cfg, maxAttempts, 1, &body,
 	)
 
 	s.Engine.ExportProcessNewTask(context.TODO(), &first)
@@ -382,7 +382,7 @@ func TestDeliveryMetrics_BreakerBlockedIsNotAnAttempt(
 		ExportDefaultFailureThreshold; attempt++ {
 		task := iTask(
 			d, event, s.WebhookID, targetID,
-			"metrics-blocked", cfg, maxRetries, attempt, &body,
+			"metrics-blocked", cfg, maxAttempts, attempt, &body,
 		)
 
 		s.Engine.ExportProcessRetryTask(context.TODO(), &task)
@@ -405,7 +405,7 @@ func TestDeliveryMetrics_BreakerBlockedIsNotAnAttempt(
 
 	blocked := iTask(
 		d, event, s.WebhookID, targetID,
-		"metrics-blocked", cfg, maxRetries,
+		"metrics-blocked", cfg, maxAttempts,
 		delivery.ExportDefaultFailureThreshold+1, &body,
 	)