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/server/alpine_browser_test.go

@@ -354,11 +354,11 @@ func checkAddEachTargetType(ctx context.Context, t *testing.T, url string) {
 	}{
 		{
 			"http", "http",
-			"csrf_token name type url headers timeout max_retries",
+			"csrf_token name type url headers timeout max_attempts",
 			map[string]string{"url": publicTargetURL},
 		},
 		{
-			"slack", "slack", "csrf_token name type url max_retries",
+			"slack", "slack", "csrf_token name type url max_attempts",
 			map[string]string{"url": publicTargetURL},
 		},
 		{
@@ -630,11 +630,11 @@ func checkRefusedEdits(
 		values map[string]string
 	}{
 		{page + "/targets/" + targetID + "/edit", map[string]string{
-			"#name":        "edited-target",
-			"#url":         "http://127.0.0.1/hook",
-			"#headers":     "X-Edited: kept",
-			"#timeout":     "12",
-			"#max_retries": "3",
+			"#name":         "edited-target",
+			"#url":          "http://127.0.0.1/hook",
+			"#headers":      "X-Edited: kept",
+			"#timeout":      "12",
+			"#max_attempts": "3",
 		}},
 		{page + "/edit", map[string]string{
 			"#name":           "edited-webhook",