fix(security): close cross-event thumbnail leak, bulk-op ownership bypass, + hardening

Auth/access-control audit fixes (all pre-existing on main; none are
regressions). Verified end-to-end where noted.

HIGH
- Thumbnail enumeration: photoAuth granted any gallery token access to any
  flat /thumbnails/thumb_* file, so a visitor to one gallery could
  enumerate another (password-protected) gallery's entire thumbnail set.
  Scope thumbnail access to the token's event via photos.thumbnail_path.
  Live-verified: cross-event fetch now 404s, own-event still 200s.
- Bulk ownership bypass: bulk-archive/bulk-delete acted on body-supplied
  event ids with no owner filter (single-event routes enforce
  requireEventOwnership), letting admin/editor archive or cascade-delete
  any event. Add filterOwnedEventIds; also guard rename + import-external;
  tighten photo-retry to scope admin (not just editor). Fix misleading
  bulk-delete comment.

MED
- verifyGalleryAccess never checked decoded.type — assert 'gallery'
  instead of relying on other token types incidentally lacking eventId.
- secure-images generate-token/secure-download missing denySlideshowToken
  (#646 bypass): a leaked slideshow token could download originals.
- Frontend: AuthenticatedImage + api.ts attached the gallery bearer token
  to absolute/external URLs — only attach to relative same-app paths.

LOW hardening
- Pin algorithms:['HS256'] on all auth-boundary jwt.verify calls.
- crypto.timingSafeEqual for share-token + HMAC compares (utils/timingSafe).
- Remove dead photoAuth import in galleryFeedback.

Tests: new regression suites for thumbnail scoping + filterOwnedEventIds;
fixed verifyGalleryAccess.customerRevoke fixture (real customer tokens
carry type:'gallery'). Full backend suite at the pre-existing baseline
(5 suites/27 tests fail on main too), zero new failures.

backend/src/middleware/sessionTimeout.js

@@ -86,7 +86,7 @@ async function sessionTimeoutMiddleware(req, res, next) {
   
   try {
     // Verify token is valid
-    const decoded = jwt.verify(token, process.env.JWT_SECRET);
+    const decoded = jwt.verify(token, process.env.JWT_SECRET, { algorithms: ['HS256'] });
     
     // Check if this is an admin token
     if (!decoded.id) {
@@ -127,7 +127,7 @@ async function sessionTimeoutMiddleware(req, res, next) {
     for (const [oldToken, _] of sessions.entries()) {
       if (oldToken !== token) {
         try {
-          const oldDecoded = jwt.verify(oldToken, process.env.JWT_SECRET);
+          const oldDecoded = jwt.verify(oldToken, process.env.JWT_SECRET, { algorithms: ['HS256'] });
           if (oldDecoded.id === userId) {
             sessions.delete(oldToken);
           }