refactor: use npm prune --omit=dev instead of deprecated --production flag

backend/Dockerfile

@@ -17,8 +17,8 @@ COPY . .
 # Build TypeScript
 RUN npm run build
 
-# Prune devDependencies after build
-RUN npm prune --production
+# Prune devDependencies after build (suppress warning)
+RUN npm prune --omit=dev
 
 # Production stage
 FROM node:20-alpine