fix: update dependencies to resolve code scanning security alerts

- Upgrade multer to 2.1.1 (CVE-2026-3520, DoS via malformed requests)
- Update tar override to >=7.5.11 (CVE-2026-31802, CVE-2026-29786)
- Upgrade Node base image from 20-alpine to 22-alpine to fix npm
  bundled tar/minimatch CVEs in the Docker image

backend/Dockerfile

@@ -1,4 +1,4 @@
-FROM node:20-alpine AS builder
+FROM node:22-alpine AS builder
 
 # Add build arguments
 ARG CACHEBUST=1
@@ -23,7 +23,7 @@ RUN npm ci --omit=dev
 COPY . .
 
 # Production stage
-FROM node:20-alpine
+FROM node:22-alpine
 
 WORKDIR /app