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_toggle_test.go

@@ -59,7 +59,7 @@ func TestHandleTargetToggle_DoesNotUndoAnEdit(t *testing.T) {
 	stored := storedTarget(t, env, tgt.ID)
 	assert.False(t, stored.Active)
 	assert.Equal(t, "edited-name", stored.Name)
-	assert.Equal(t, 5, stored.MaxRetries)
+	assert.Equal(t, 5, stored.MaxAttempts)
 	assert.Equal(
 		t, editReplacedURL, storedHTTPConfig(t, env, tgt.ID).URL,
 	)