fix: resolve password change redirect loop and file watcher crash

#263: The mandatory password change modal updated React state before
the browser stored the new JWT cookie, causing a race condition where
the auth context checked the session with the old (invalidated) token.
Replace the state update with a full page redirect to /admin/dashboard
after a brief delay, ensuring the new cookie is applied cleanly.

#269: The file watcher service imported isVideoMimeType from
fileSecurityUtils where it doesn't exist. The function is exported
from videoProcessor. Fix the import path.

Closes #269

backend/src/services/fileWatcher.js

@@ -5,7 +5,7 @@ const { db } = require('../database/db');
 const { formatBoolean } = require('../utils/dbCompat');
 const { generateThumbnail, generateVideoPlaceholder } = require('./imageProcessor');
 const logger = require('../utils/logger');
-const { isVideoMimeType } = require('../utils/fileSecurityUtils');
+const { isVideoMimeType } = require('./videoProcessor');
 const mime = require('mime-types');
 
 const getStoragePath = () => process.env.STORAGE_PATH || path.join(__dirname, '../../../storage');