WCAG 4.1.2: add WebVTT caption support for <video> elements on tfe.php

Problem: <video> elements on tfe.php had no <track kind="captions"> element,
violating WCAG 4.1.2 (name, role, value) for video content.

Changes:
- public/tfe.php: collect all text/vtt files from the thesis file list before
  rendering; skip standalone rendering of .vtt entries; for each MP4 emit a
  <track kind="captions" srclang="fr" label="Sous-titres" default> pointing
  to the N-th VTT file (N-th video paired with N-th caption in document order)
- public/media.php: add text/vtt to allowed MIME list; normalise finfo
  text/plain -> text/vtt for .vtt files; add vtt branch to cache/header
  block (Content-Type: text/vtt; charset=utf-8, 1-day cache)
- public/admin/actions/formulaire.php: allow .vtt uploads (text/vtt MIME,
  vtt extension); normalise text/plain finfo result; set file_type='caption'
  for VTT files so they are distinguishable from other thesis files
- public/admin/add.php: extend files field accept attr to include .vtt;
  update hint text to document the VTT sidecar convention

VTT files uploaded under theses/ inherit the same access_type visibility
gate in media.php as all other thesis content (403 for access_type_id=3).

public/admin/add.php

@@ -94,7 +94,7 @@ function wasSelected($key, $value) {
 
         <?php $name = 'banner'; $label = 'Image bannière (accueil) :'; $accept = 'image/jpeg,image/png,image/webp'; $hint = 'JPG, PNG ou WEBP. Format paysage recommandé (4:1). Max 5 MB.'; include APP_ROOT . '/templates/partials/form/file-field.php'; ?>
 
-        <?php $name = 'files'; $label = 'Fichiers du TFE :'; $accept = '.pdf,.jpg,.jpeg,.png,.mp4,.zip'; $hint = 'PDF, JPG, PNG, MP4, ZIP. Max 50 MB par fichier.'; $multiple = true; include APP_ROOT . '/templates/partials/form/file-field.php'; ?>
+        <?php $name = 'files'; $label = 'Fichiers du TFE :'; $accept = '.pdf,.jpg,.jpeg,.png,.mp4,.zip,.vtt'; $hint = 'PDF, JPG, PNG, MP4, ZIP. Max 50 MB par fichier. Pour les vidéos, un fichier .vtt de sous-titres peut être joint (il sera associé automatiquement à la vidéo correspondante).'; $multiple = true; include APP_ROOT . '/templates/partials/form/file-field.php'; ?>
 
         <div class="admin-form-footer">
             <button type="submit" name="go" class="admin-btn">Soumettre</button>