feat: token refresh, video upload, playlist routes complets

- auth: cookie od_token 7j, refresh token od_refresh 30j, POST /api/auth/refresh, GET /api/auth/me/optional
- admin: POST /api/admin/videos/upload via multer (mp4/webm, 4Go max, UUID filename)
- playlist: PATCH /:id, DELETE /:id, POST /:id/videos, DELETE /:id/videos/:videoId
- env: UPLOADS_DIR documenté dans .env.example

backend/.env.example

@@ -14,3 +14,6 @@ SUPER_OAUTH_URL=https://superoauth.tetardtek.com
 
 # CORS — URL du frontend autorisé
 FRONTEND_URL=http://localhost:5173
+
+# Dossier de stockage des vidéos uploadées (défaut: ./uploads)
+UPLOADS_DIR=./uploads