Re-shared external storage (SMB) not always rescanned for recipients

Steps to reproduce 1. User A adds an SMB external storage mount. 2. User A shares a folder from that mount directly with User B. 3. Something changes on the SMB server outside of Nextcloud. 4. User A browses the folder → gets rescanned correctly. 5. User B browses the re-share → does not get rescanned (stale listing). Root cause SharedStorage::getWatcher() decides between a real Watcher and a NullWatcher purely from string metadata on the share's cache entry: getNodeCacheEntry() returns null, or a CacheEntry missing the storage_string_id key, for several legitimate code paths: • DefaultShareProvider::getSharesBy() / getShareById() / getSharesByPath() never populate a node cache entry at all (no filecache/storages join). • Even DefaultShareProvider::_getSharedWith() (used for the normal getMountsForUser/getMountsForPath mount setup) can come back without storage_string_id if the source node wasn't in filecache yet at share-fetch time. Whenever tha…