Implement TFE file access restriction feature (complete)

Requirements:
- parametres.php toggle: 'restricted_files_enabled' enables/disables the feature
- Public TFE page: when enabled + access_type=Interne, hides files, shows French
  restriction message + access request form (metadata/synopsis still visible)
- ERG emails (@erg.school / @erg.be): auto-approve, send 24h access link immediately
- External emails: show justification textarea, create pending request, notify admin
- Admin panel /admin/file-access.php: approve/reject requests with optional notes,
  sends access email on approval (linked from admin nav with pending count badge)

Security:
- One-time 24h email tokens (used_at + is_valid=0 on first click)
- Token redeemed via POST /validate-access (GET shows confirmation page only)
- Long-lived 30-day browser session in file_access_sessions table
- Cookie: HttpOnly + Secure + SameSite=Strict
- CSRF on all mutations, rate limiting on request submission
- Audit trail: IP, UA, event, timestamp in file_access_audit

Bug fixes:
- admin/file-access.php: $vars never extract()ed → page was blank
- Template had self-contained head/footer includes (double-include)
- Admin approval URL used $requestId instead of $request['thesis_id']
- App::boot() now starts session so CSRF token works on public pages
- Dispatcher routes /validate-access and /request-access through front controller

app/templates/admin/parametres.php

@@ -102,6 +102,19 @@
                 </label>
             </fieldset>
 
+            <fieldset>
+                <legend>Restriction d'accès aux fichiers</legend>
+
+                <label class="param-checkbox">
+                    <input type="checkbox" name="restricted_files_enabled" value="1"
+                           <?= ($siteSettings['restricted_files_enabled'] ?? '0') === '1' ? 'checked' : '' ?>>
+                    <span>
+                        <strong>Activer la restriction d'accès</strong><br>
+                        <small>Pour les TFE de type "Interne", masquer les fichiers et exiger une demande d'accès par email. Les métadonnées et le résumé restent visibles publiquement.</small>
+                    </span>
+                </label>
+            </fieldset>
+
             <button type="submit">Enregistrer</button>
         </form>
     </section>