chore: bump golang from 1.25 to 1.26 in /backend

Bumps golang from 1.25 to 1.26.

---
updated-dependencies:
- dependency-name: golang
  dependency-version: '1.26'
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

backend/Dockerfile

@@ -1,6 +1,6 @@
 # syntax=docker/dockerfile:1
 
-FROM golang:1.25 AS builder
+FROM golang:1.26 AS builder
 
 WORKDIR /app