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/src/Controllers/ThesisFileHandler.php

@@ -6,7 +6,7 @@
  * Shared file-upload logic used by ThesisCreateController and
  * ThesisEditController.  All on-disk files are stored under:
  *
- *   theses/{YYYY}/{YYYY}_{AUTHORS}_{TITLE_SLUG}/
+ *   documents/{YYYY}/{YYYY}_{AUTHORS}_{TITLE_SLUG}/
  *
  * Filenames follow:
  *
@@ -88,7 +88,7 @@ trait ThesisFileHandler
      *
      * @param int        $thesisId
      * @param array|null $upload     Single-file $_FILES entry.
-     * @param string     $folderPath Relative path from STORAGE_ROOT to the thesis folder (e.g. "theses/2025/2025_SMITH_Mon_Titre/").
+     * @param string     $folderPath Relative path from STORAGE_ROOT to the thesis folder (e.g. "documents/2025/2025_SMITH_Mon_Titre/").
      * @param string     $filePrefix The prefix shared by all files in this folder (e.g. "2025_SMITH_Mon_Titre").
      */
     protected function handleCoverUpload(int $thesisId, ?array $upload, string $folderPath, string $filePrefix): void
@@ -802,7 +802,7 @@ trait ThesisFileHandler
     /**
      * Build the folder path and file prefix for a thesis.
      *
-     * Folder:  theses/{YYYY}/{YYYY}_{AUTHORS}_{TITLE_SLUG}/
+     * Folder:  documents/{YYYY}/{YYYY}_{AUTHORS}_{TITLE_SLUG}/
      * Prefix:  {YYYY}_{AUTHORS}_{TITLE_SLUG}
      *
      * @return array{folderPath: string, filePrefix: string, folderName: string}
@@ -814,7 +814,7 @@ trait ThesisFileHandler
 
         $folderName = $year . '_' . $authorSlug . '_' . $titleSlug;
         $filePrefix = $folderName;
-        $folderPath = 'theses/' . $year . '/' . $folderName . '/';
+        $folderPath = 'documents/' . $year . '/' . $folderName . '/';
 
         return [
             'folderPath' => $folderPath,
@@ -1203,4 +1203,55 @@ trait ThesisFileHandler
         }
         @rmdir($tmpDir);
     }
+
+    /**
+     * Delete a thesis_files row, moving the on-disk file to the trash directory
+     * instead of unlinking it. Website URLs and PeerTube references are only
+     * removed from the database (no disk file).
+     */
+    protected function deleteThesisFileToTrash(int $fileId, int $thesisId): void
+    {
+        if ($fileId <= 0) {
+            return;
+        }
+
+        // Fetch the file row to get the path before deleting the DB record
+        $existingFiles = $this->db->getThesisFiles($thesisId);
+        $fileRow = null;
+        foreach ($existingFiles as $f) {
+            if ((int)$f['id'] === $fileId) {
+                $fileRow = $f;
+                break;
+            }
+        }
+
+        $filePath = $this->db->deleteThesisFile($fileId, $thesisId);
+
+        if ($filePath && defined('STORAGE_ROOT')) {
+            // Skip filesystem for website URLs and PeerTube IDs (not real files)
+            if (str_starts_with($filePath, 'http://') || str_starts_with($filePath, 'https://')) {
+                return;
+            }
+            if (str_starts_with($filePath, 'peertube_ids:')) {
+                return;
+            }
+
+            $abs = STORAGE_ROOT . '/' . $filePath;
+            if (file_exists($abs)) {
+                $trashDir = STORAGE_ROOT . '/tmp/_trash';
+                if (!is_dir($trashDir)) {
+                    mkdir($trashDir, 0755, true);
+                }
+                // Keep original filename structure for traceability
+                $trashName = $fileId . '_' . basename($filePath);
+                $trashPath = $trashDir . '/' . $trashName;
+                if (!rename($abs, $trashPath)) {
+                    // Fallback to unlink if rename fails (cross-device)
+                    @copy($abs, $trashPath);
+                    @unlink($abs);
+                }
+                error_log("ThesisFileHandler: file \$fileId moved to trash → \$trashName");
+            }
+        }
+    }
 }