Fix npm ci command for newer npm versions

Replace deprecated --only=production with --omit=dev flag
which is required for npm 10+ after the npm upgrade.

backend/Dockerfile

@@ -19,8 +19,8 @@ WORKDIR /app
 # Copy package files
 COPY package*.json ./
 
-# Install dependencies
-RUN npm ci --only=production
+# Install dependencies (--omit=dev replaces deprecated --only=production)
+RUN npm ci --omit=dev
 
 # Copy application files
 COPY . .