fix admin texts route shadow conflict

FastRoute rejected /api/texts/all because the previously
declared variable route /api/texts/{textId} would shadow it,
crashing the app on boot. move the admin all-texts endpoint
to /api/admin/texts to clear the conflict; admin texts.js
follows the new URL.

public/js/texts.js

@@ -3,7 +3,7 @@ document.addEventListener('DOMContentLoaded', () => {
     const form = document.getElementById('texts-form');
 
     async function loadTexts() {
-        const res = await fetch('/api/texts/all', {
+        const res = await fetch('/api/admin/texts', {
             credentials: 'same-origin',
         });
         const texts = await res.json();