fix: use npm install instead of npm ci for more resilient builds

npm ci fails when package-lock.json is out of sync with package.json,
which is common with GitHub-downloaded ZIPs. Switched to npm install
with --legacy-peer-deps flag for maximum compatibility.

backend/src/build/build.service.ts

@@ -293,7 +293,7 @@ export class BuildService {
 FROM node:20-alpine AS builder
 WORKDIR /app
 COPY package*.json ./
-RUN if [ -f package-lock.json ]; then npm ci; else npm install; fi && npm cache clean --force
+RUN npm install --legacy-peer-deps && npm cache clean --force
 COPY . .
 
 # Auto-detect Next.js and enable standalone output