fix: update builder image to golang:1.25-alpine

go.mod requires Go 1.25+, but Dockerfile was still using golang:1.22-alpine
as the builder image. This caused CI build failure:
  "go.mod requires go >= 1.25.0 (running go 1.22.12)"

Dockerfile

@@ -1,7 +1,7 @@
 # Multi-stage build for MrixsCraft backend.
 # Final image: ~20 MB, non-root, no toolchain.
 
-FROM golang:1.22-alpine AS builder
+FROM golang:1.25-alpine AS builder
 WORKDIR /build
 COPY go.mod go.sum ./
 RUN go mod download