Add spelling test polish: date filter, CSV export, student history, planner badges

- Gradebook: period dropdown (all time / 30 / 90 days) and CSV export for
  Spelling Test History section
- Student portal spelling tab: shows last 10 tests as a Recent Tests list
- Planner: completed standalone spelling tests appear as read-only ⭐ badges
  on the day they were taken (week and month views)
- Removed dead s.tests query from /api/overview (now served by dedicated endpoints)
- New endpoints: GET /api/spelling-tests/csv, GET /api/students/:id/tests,
  GET /api/admin/students/:id/tests-range

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

public/admin.html

@@ -284,6 +284,14 @@
         </div>
         <div id="gradebook-table"></div>
         <h2>Spelling Test History</h2>
+        <div class="inline-form">
+          <label>Period <select id="spelling-tests-since">
+            <option value="">All time</option>
+            <option value="30">Last 30 days</option>
+            <option value="90">Last 90 days</option>
+          </select></label>
+          <button id="spelling-tests-csv-btn" class="secondary small">⬇ CSV</button>
+        </div>
         <div id="spelling-tests-table"></div>
         <div id="history-modal" class="modal" hidden>
           <div class="modal-box">