update backend building

backend/Dockerfile

@@ -15,6 +15,9 @@ COPY package*.json ./
 # Install all dependencies (including devDependencies for building)
 RUN npm ci
 
+# Expose local binaries (e.g. tsc) to the shell PATH so npm run build resolves them
+ENV PATH="/app/node_modules/.bin:$PATH"
+
 # Copy TypeScript configuration and source code
 COPY tsconfig.json ./
 COPY src ./src