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/handlers/target_edit_test.go

@@ -117,7 +117,7 @@ func seedHTTPTarget(
 	form.Set("url", editOriginalURL)
 	form.Set("headers", headers)
 	form.Set("timeout", timeout)
-	form.Set("max_retries", "3")
+	form.Set("max_attempts", "3")
 
 	w := serveTarget(
 		env, http.MethodPost,
@@ -179,7 +179,7 @@ func editForm(targetURL, headers, timeout string) url.Values {
 	form.Set("url", targetURL)
 	form.Set("headers", headers)
 	form.Set("timeout", timeout)
-	form.Set("max_retries", "5")
+	form.Set("max_attempts", "5")
 
 	return form
 }
@@ -280,9 +280,9 @@ func TestHandleTargetEditSubmit_Saves(t *testing.T) {
 		assertEditClearingRemovesThem(t, env)
 	})
 
-	t.Run("absent max_retries is not zeroed", func(t *testing.T) {
+	t.Run("absent max_attempts is kept", func(t *testing.T) {
 		t.Parallel()
-		assertEditKeepsAbsentMaxRetries(t, env)
+		assertEditKeepsAbsentMaxAttempts(t, env)
 	})
 }
 
@@ -307,7 +307,7 @@ func assertEditChangesDestination(
 
 	reloaded := storedTarget(t, env, target.ID)
 	assert.Equal(t, "edited-name", reloaded.Name)
-	assert.Equal(t, 5, reloaded.MaxRetries)
+	assert.Equal(t, 5, reloaded.MaxAttempts)
 	assert.Equal(
 		t, database.TargetTypeHTTP, reloaded.Type,
 		"the edit form must not change a target's type",
@@ -368,25 +368,25 @@ func assertEditClearingRemovesThem(
 	assert.Zero(t, cfg.Timeout)
 }
 
-// Retries are offered only by the forms for target types that retry.
-// An absent field means the form does not edit retries, not that
-// they should be turned off.
-func assertEditKeepsAbsentMaxRetries(
+// Delivery attempts are offered only by the forms for target types
+// that retry. An absent field means the form does not edit them, not
+// that retries should be turned off.
+func assertEditKeepsAbsentMaxAttempts(
 	t *testing.T, env *sourceTestEnv,
 ) {
 	t.Helper()
 
 	webhook, target := seedHTTPTarget(t, env, "", "")
-	require.Equal(t, 3, target.MaxRetries)
+	require.Equal(t, 3, target.MaxAttempts)
 
 	form := editForm(editOriginalURL, "", "")
-	form.Del("max_retries")
+	form.Del("max_attempts")
 
 	w := submitTargetEdit(env, webhook.ID, target.ID, form)
 	require.Equal(t, http.StatusSeeOther, w.Code, w.Body.String())
 
 	assert.Equal(
-		t, 3, storedTarget(t, env, target.ID).MaxRetries,
+		t, 3, storedTarget(t, env, target.ID).MaxAttempts,
 	)
 }
 
@@ -659,12 +659,12 @@ func TestHandleTargetEditSubmit_RefusedFormComesBack(t *testing.T) {
 		{
 			database.TargetTypeHTTP,
 			"name=edited&url=" + editBlockedURL +
-				"&headers=X-Edited:+kept&timeout=12&max_retries=3",
+				"&headers=X-Edited:+kept&timeout=12&max_attempts=3",
 			"Invalid target URL",
 		},
 		{
 			database.TargetTypeSlack,
-			"name=edited&url=" + editOriginalURL + "&max_retries=25",
+			"name=edited&url=" + editOriginalURL + "&max_attempts=25",
 			"Invalid delivery attempts",
 		},
 		{