Move spelling test results from Spelling panel into Gradebook

Adds a GET /api/spelling-tests endpoint that returns all standalone test
results grouped by student. The Gradebook panel now shows a "Spelling Test
History" section below the course gradebook table. Removed the "Test results"
section from the Spelling panel left column.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/admin.html

@@ -283,6 +283,8 @@
           <button id="gradebook-csv-btn" class="secondary">⬇ Export CSV</button>
         </div>
         <div id="gradebook-table"></div>
+        <h2>Spelling Test History</h2>
+        <div id="spelling-tests-table"></div>
         <div id="history-modal" class="modal" hidden>
           <div class="modal-box">
             <h3 id="history-modal-title">Attempt History</h3>
@@ -305,8 +307,6 @@
               <button type="button" id="list-show-hidden" class="secondary small" hidden>Show hidden</button>
             </div>
             <div id="list-rows"></div>
-            <h2>Test results</h2>
-            <div id="results-area"></div>
           </div>
           <div class="panel-col">
             <div class="toggle-btns">