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/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",
 		},
 		{