fix: bump Docker base image to golang:1.25-alpine

go.mod now requires go 1.25.0 (bumped by Dependabot). The 1.23-alpine
builder was trying to auto-download the 1.25 toolchain at build time,
which fails in network-restricted environments.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: johnzastrow

docker/Dockerfile

@@ -23,7 +23,7 @@ RUN npm run build
 # ============================================================================
 # Stage 2: Build Backend (Go)
 # ============================================================================
-FROM golang:1.23-alpine AS backend-builder
+FROM golang:1.25-alpine AS backend-builder
 
 # Install build dependencies
 RUN apk add --no-cache git gcc musl-dev sqlite-dev