chore(deps): bump node from 20-alpine to 26-alpine in /backend

Bumps node from 20-alpine to 26-alpine.

---
updated-dependencies:
- dependency-name: node
  dependency-version: 26-alpine
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

backend/Dockerfile

@@ -1,5 +1,5 @@
 # Stage 1: Build
-FROM node:20-alpine AS builder
+FROM node:26-alpine AS builder
 
 WORKDIR /app
 
@@ -10,7 +10,7 @@ COPY . .
 RUN npm run build
 
 # Stage 2: Production
-FROM node:20-alpine AS production
+FROM node:26-alpine AS production
 
 WORKDIR /app