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/ThesisCreateController.php

@@ -191,7 +191,7 @@ class ThesisCreateController
         $tf = $this->buildThesisFolder($data['annee'], $allAuthorsStr, $data['titre']);
         $folderName = $this->ensureUniqueFolder($tf['folderPath']);
         // Rebuild path with potentially modified folder name
-        $folderPath = 'theses/' . $data['annee'] . '/' . $folderName . '/';
+        $folderPath = 'documents/' . $data['annee'] . '/' . $folderName . '/';
         $filePrefix = $folderName;
 
         if (!empty($post['filepond_mode'])) {