Caddy 2.10.2 requires Go 1.25 (declared in its go.mod). The previous
commit incorrectly downgraded to Go 1.23 based on the false assumption
that Go 1.25.5 doesn't exist.
This fix:
- Updates Dockerfile Go images from 1.23-alpine to 1.25-alpine
- Updates backend/go.mod to go 1.25
- Updates go.work to go 1.25
Fixes CI Docker build failures in xcaddy stage.
