perf(admin): infinite-scroll the contenus langues/mots-clés tables

The contenus page loaded both lookup tables in full via HTMX on page load:
760 tag rows + 216 language rows, ~2.4MB of HTML including ~1960 inline
SVG icons and per-row CSRF forms. The DB queries were already fast (~6ms);
the cost was pure client-side payload and DOM.

- Add paged lookups: getTagsPage/getLanguagesPage + countTagsWithCount/
  countLanguagesWithCount, and optional limit/offset on the existing
  unpaged variants (backward compatible).
- Fragments serve 25 rows per request. A sentinel <tr> with
  hx-trigger="intersect once root:#<wrap>" appends the next page when it
  scrolls into the table's own scroll container (htmx 'revealed' only
  checks the window viewport, not nested scrollers).
- Search forms still swap the whole wrapper and reset to offset 0.
- Style the load-more row; add PagedLanguagesTagsTest coverage.

Initial DOM for the page drops from ~2.4MB to ~160KB; scrolling reaches the
full totals (736 tags, 216 languages) and stops cleanly.

app/public/admin/contenus-motscles-fragment.php

@@ -4,6 +4,15 @@
  *
  * HTMX fragment: returns the mots-clés table for the contenus page,
  * optionally filtered by a search query.
+ *
+ * Infinite scroll: rows are served in pages of PAGE_SIZE. When there are
+ * more rows, a sentinel <tr> is emitted with hx-trigger="revealed": when it
+ * scrolls into view htmx fetches the next offset and swaps itself
+ * (outerHTML) with the next chunk of rows + a new sentinel. This keeps the
+ * initial payload small instead of shipping all tags at once.
+ *
+ * When $offset > 0 the fragment returns only the <tr> rows (append mode),
+ * because the sentinel lives inside the existing <tbody>.
  */
 require_once __DIR__ . '/../../bootstrap.php';
 require_once __DIR__ . '/../../src/AdminAuth.php';
@@ -15,14 +24,83 @@ if (empty($_SESSION['csrf_token'])) {
 
 require_once __DIR__ . '/../../src/Database.php';
 
+const MOTSCLES_PAGE_SIZE = 25;
+
 $searchQuery = trim($_GET['q'] ?? '');
+$offset      = max(0, (int) ($_GET['offset'] ?? 0));
 
 try {
-    $db   = new Database();
-    $tags = ($searchQuery !== '') ? $db->searchTags($searchQuery) : $db->getAllTagsWithCount();
+    $db    = new Database();
+    $tags  = $db->getTagsPage($searchQuery, MOTSCLES_PAGE_SIZE, $offset);
+    $total = $db->countTagsWithCount($searchQuery);
 } catch (Exception $e) {
     die('<div class="flash-error">Erreur : ' . htmlspecialchars($e->getMessage()) . '</div>');
 }
+
+$hasMore = ($offset + count($tags)) < $total;
+$nextUrl = '/admin/contenus-motscles-fragment.php?offset=' . ($offset + MOTSCLES_PAGE_SIZE)
+         . ($searchQuery !== '' ? '&q=' . urlencode($searchQuery) : '');
+
+/**
+ * Render one <tr> for a tag row.
+ */
+function render_motscles_row(array $tag): void
+{
+    ?>
+        <tr>
+            <td style="width:1%"><input type="checkbox" name="selected_tags[]" value="<?= (int)$tag['id'] ?>" onchange="motsclesUpdateBulk()"></td>
+            <td id="motscles-name-<?= (int)$tag['id'] ?>" data-name="<?= htmlspecialchars($tag['name']) ?>">
+                <span class="tag-name-cell"><?= htmlspecialchars($tag['name']) ?></span>
+                <button type="button" class="admin-icon-btn admin-icon-btn--edit" title="Renommer"
+                        onclick="motsclesStartRename(<?= (int)$tag['id'] ?>)">
+                    <?= icon('pencil-note') ?>
+                </button>
+            </td>
+            <td class="admin-tags-count" style="width:1%;white-space:nowrap"><?= (int)$tag['thesis_count'] ?></td>
+            <td class="admin-actions-col" style="width:1%">
+                <div class="admin-actions">
+                    <form method="post" action="actions/tag.php" class="admin-inline-form">
+                        <input type="hidden" name="csrf_token" value="<?= htmlspecialchars($_SESSION['csrf_token']) ?>">
+                        <input type="hidden" name="action" value="delete">
+                        <input type="hidden" name="return" value="/admin/contenus.php">
+                        <input type="hidden" name="tag_id" value="<?= (int)$tag['id'] ?>">
+                        <button type="button" class="admin-icon-btn admin-icon-btn--delete" title="Supprimer"
+                                onclick="motsclesConfirmDelete(this, <?= htmlspecialchars(json_encode($tag['name']), ENT_QUOTES) ?>)">
+                            <?= icon('trash') ?>
+                        </button>
+                    </form>
+                </div>
+            </td>
+        </tr>
+    <?php
+}
+
+/**
+ * Render the "load more" sentinel row (or nothing when exhausted).
+ */
+function render_motscles_sentinel(bool $hasMore, string $nextUrl): void
+{
+    if (!$hasMore) {
+        return;
+    }
+    ?>
+        <tr class="admin-load-more" hx-get="<?= htmlspecialchars($nextUrl) ?>"
+            hx-trigger="intersect once root:#motscles-table-wrap" hx-swap="outerHTML" hx-target="this">
+            <td colspan="4" class="admin-load-more__cell">Chargement…</td>
+        </tr>
+    <?php
+}
+
+// ── Append mode: only the rows, swapped in place of the sentinel ──────────
+if ($offset > 0) {
+    foreach ($tags as $tag) {
+        render_motscles_row($tag);
+    }
+    render_motscles_sentinel($hasMore, $nextUrl);
+    return;
+}
+
+// ── Initial load: full wrapper ────────────────────────────────────────────
 ?>
 <div id="motscles-bulk-actions" class="admin-bulk-actions" style="display:none;position:sticky;top:0;z-index:10">
     <strong><span id="motscles-selected-count">0</span> mot(s)-clé(s) sélectionné(s)</strong>
@@ -68,32 +146,9 @@ try {
         <tr><td colspan="4" class="admin-empty">Aucun mot-clé trouvé.</td></tr>
         <?php else: ?>
         <?php foreach ($tags as $tag): ?>
-        <tr>
-            <td style="width:1%"><input type="checkbox" name="selected_tags[]" value="<?= (int)$tag['id'] ?>" onchange="motsclesUpdateBulk()"></td>
-            <td id="motscles-name-<?= (int)$tag['id'] ?>" data-name="<?= htmlspecialchars($tag['name']) ?>">
-                <span class="tag-name-cell"><?= htmlspecialchars($tag['name']) ?></span>
-                <button type="button" class="admin-icon-btn admin-icon-btn--edit" title="Renommer"
-                        onclick="motsclesStartRename(<?= (int)$tag['id'] ?>)">
-                    <?= icon('pencil-note') ?>
-                </button>
-            </td>
-            <td class="admin-tags-count" style="width:1%;white-space:nowrap"><?= (int)$tag['thesis_count'] ?></td>
-            <td class="admin-actions-col" style="width:1%">
-                <div class="admin-actions">
-                    <form method="post" action="actions/tag.php" class="admin-inline-form">
-                        <input type="hidden" name="csrf_token" value="<?= htmlspecialchars($_SESSION['csrf_token']) ?>">
-                        <input type="hidden" name="action" value="delete">
-                        <input type="hidden" name="return" value="/admin/contenus.php">
-                        <input type="hidden" name="tag_id" value="<?= (int)$tag['id'] ?>">
-                        <button type="button" class="admin-icon-btn admin-icon-btn--delete" title="Supprimer"
-                                onclick="motsclesConfirmDelete(this, <?= htmlspecialchars(json_encode($tag['name']), ENT_QUOTES) ?>)">
-                            <?= icon('trash') ?>
-                        </button>
-                    </form>
-                </div>
-            </td>
-        </tr>
+        <?php render_motscles_row($tag); ?>
         <?php endforeach; ?>
+        <?php render_motscles_sentinel($hasMore, $nextUrl); ?>
         <?php endif; ?>
     </tbody>
 </table>