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/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",