feat: fix file deletion on save + trash policy + documents/ prefix + relink browser

1. note_intention: Delete old file only when a genuinely new upload arrives
   (32-char hex file_id), not when the FilePond pool preserves an existing
   file by sending its DB integer ID.  Previously the DB integer ID
   triggered $hasNewNote=true, which deleted the existing note_intention
   from disk+DB, then handleFilePondSingleFile couldn't re-process it
   because the regex requires a hex pattern.  Same fix applied to cover.

2. All file deletions now use deleteThesisFileToTrash() which renames
   files to tmp/_trash/ instead of unlinking.  The trash preserves
   original filenames prefixed with DB id for traceability.  Skips
   website URLs and PeerTube refs (no disk file).

3. Storage prefix changed from theses/ to documents/ to reflect that
   the folder holds all document types (determined by file_type in DB).
   MediaController visibility gate supports both prefixes for backward
   compat with existing files.

4. File browser + relink feature for orphaned files:
   - /admin/fragments/file-browser.php — HTMX tree browser for
     storage/documents/ and storage/theses/
   - /admin/actions/filepond/relink.php — POST endpoint that inserts
     a thesis_files row pointing to existing on-disk file
   - Per-pool "📂 Relier" buttons (edit mode only)
   - JS: XamxamOpenFileBrowser / XamxamRelinkFile with FilePond integration
   - CSS: .relink-modal dialog + .file-browser tree styles

app/public/admin/fragments/file-browser.php

@@ -0,0 +1,146 @@
+<?php
+/**
+ * File browser fragment — returns a clickable directory tree of documents/ + theses/.
+ *
+ * GET /admin/fragments/file-browser.php?dir=documents/2025
+ *
+ * Used by the relink modal to let admins pick an orphaned file and reattach
+ * it to a thesis entry.
+ */
+require_once __DIR__ . '/../../../bootstrap.php';
+require_once __DIR__ . '/../../../src/AdminAuth.php';
+AdminAuth::requireLogin();
+
+$storageRoot = STORAGE_ROOT;
+
+// Determine which directory to browse
+$relDir = trim($_GET['dir'] ?? '', '/');
+if ($relDir !== '' && !preg_match('#^(documents|theses)/#', $relDir)) {
+    $relDir = '';
+}
+
+$currentAbs = $storageRoot . '/' . ($relDir !== '' ? $relDir . '/' : '');
+// Security: prevent escaping STORAGE_ROOT
+$realCurrent = realpath($currentAbs);
+$realStorage = realpath($storageRoot);
+if ($realCurrent === false || !str_starts_with($realCurrent, $realStorage)) {
+    http_response_code(403);
+    die('Chemin interdit.');
+}
+
+// Build breadcrumb
+$breadcrumb = [];
+if ($relDir !== '') {
+    $parts = explode('/', $relDir);
+    $accum = '';
+    foreach ($parts as $p) {
+        $accum = ltrim($accum . '/' . $p, '/');
+        $breadcrumb[] = ['label' => $p, 'dir' => $accum];
+    }
+}
+
+// Gather entries
+$entries = [];
+if (is_dir($realCurrent)) {
+    $dh = opendir($realCurrent);
+    if ($dh) {
+        while (($name = readdir($dh)) !== false) {
+            if ($name === '.' || $name === '..') continue;
+            $full = $realCurrent . '/' . $name;
+            $isDir = is_dir($full);
+            $entries[] = [
+                'name'   => $name,
+                'is_dir' => $isDir,
+                'size'   => $isDir ? null : filesize($full),
+                'ext'    => $isDir ? null : strtolower(pathinfo($name, PATHINFO_EXTENSION)),
+            ];
+        }
+        closedir($dh);
+    }
+}
+
+// Sort: dirs first, then files, alphabetical
+usort($entries, function ($a, $b) {
+    if ($a['is_dir'] !== $b['is_dir']) return $a['is_dir'] ? -1 : 1;
+    return strnatcasecmp($a['name'], $b['name']);
+});
+
+// Human-readable filesize
+function fmtSize(?int $bytes): string {
+    if ($bytes === null) return '';
+    if ($bytes >= 1_000_000_000) return round($bytes / 1_000_000_000, 1) . ' GB';
+    if ($bytes >= 1_000_000) return round($bytes / 1_000_000, 1) . ' MB';
+    if ($bytes >= 1_000) return round($bytes / 1_000, 1) . ' KB';
+    return $bytes . ' B';
+}
+
+// Determine parent dir
+$parentDir = '';
+if ($relDir !== '') {
+    $parentParts = explode('/', $relDir);
+    array_pop($parentParts);
+    $parentDir = implode('/', $parentParts);
+}
+
+$rootDirs = ['documents', 'theses'];
+?>
+<div id="file-browser-container" class="file-browser">
+    <?php if ($relDir === ''): ?>
+        <!-- Root: show top-level directories -->
+        <p class="file-browser-hint">Sélectionnez un dossier pour parcourir les fichiers orphelins.</p>
+        <ul class="file-browser-list">
+            <?php foreach ($rootDirs as $rd): ?>
+                <?php $rdAbs = $storageRoot . '/' . $rd; ?>
+                <li class="file-browser-entry file-browser-dir">
+                    <a href="#" hx-get="/admin/fragments/file-browser.php?dir=<?= urlencode($rd) ?>"
+                       hx-target="#file-browser-container" hx-swap="outerHTML">
+                        <span class="file-browser-icon">📁</span>
+                        <span class="file-browser-name"><?= htmlspecialchars($rd) ?>/</span>
+                    </a>
+                </li>
+            <?php endforeach; ?>
+        </ul>
+    <?php else: ?>
+        <!-- Subdirectory: breadcrumb + entries -->
+        <nav class="file-browser-breadcrumb">
+            <a href="#" hx-get="/admin/fragments/file-browser.php"
+               hx-target="#file-browser-container" hx-swap="outerHTML">📂 racine</a>
+            <?php foreach ($breadcrumb as $i => $bc): ?>
+                <span class="file-browser-sep">/</span>
+                <a href="#" hx-get="/admin/fragments/file-browser.php?dir=<?= urlencode($bc['dir']) ?>"
+                   hx-target="#file-browser-container" hx-swap="outerHTML"><?= htmlspecialchars($bc['label']) ?></a>
+            <?php endforeach; ?>
+        </nav>
+
+        <?php if (empty($entries)): ?>
+            <p class="file-browser-empty">Ce dossier est vide.</p>
+        <?php else: ?>
+            <ul class="file-browser-list">
+                <?php foreach ($entries as $e): ?>
+                    <?php if ($e['is_dir']): ?>
+                        <li class="file-browser-entry file-browser-dir">
+                            <a href="#" hx-get="/admin/fragments/file-browser.php?dir=<?= urlencode($relDir . '/' . $e['name']) ?>"
+                               hx-target="#file-browser-container" hx-swap="outerHTML">
+                                <span class="file-browser-icon">📁</span>
+                                <span class="file-browser-name"><?= htmlspecialchars($e['name']) ?>/</span>
+                            </a>
+                        </li>
+                    <?php else: ?>
+                        <li class="file-browser-entry file-browser-file"
+                            data-file-path="<?= htmlspecialchars($relDir . '/' . $e['name']) ?>"
+                            data-file-name="<?= htmlspecialchars($e['name']) ?>"
+                            data-file-ext="<?= htmlspecialchars($e['ext'] ?? '') ?>"
+                            data-file-size="<?= (int)($e['size'] ?? 0) ?>">
+                            <button type="button" class="file-browser-select-btn"
+                                    onclick="XamxamRelinkFile(this)">
+                                <span class="file-browser-icon">📄</span>
+                                <span class="file-browser-name"><?= htmlspecialchars($e['name']) ?></span>
+                                <span class="file-browser-size"><?= htmlspecialchars(fmtSize($e['size'])) ?></span>
+                            </button>
+                        </li>
+                    <?php endif; ?>
+                <?php endforeach; ?>
+            </ul>
+        <?php endif; ?>
+    <?php endif; ?>
+</div>

app/public/admin/fragments/pill-search.php

@@ -0,0 +1,10 @@
+<?php
+/**
+ * Admin fragment: generic pill-search (HTMX partial).
+ */
+require_once __DIR__ . '/../../../bootstrap.php';
+require_once __DIR__ . '/../../../src/AdminAuth.php';
+AdminAuth::requireLogin();
+App::boot();
+
+require_once APP_ROOT . '/public/partage/pill-search-fragment.php';