fix(api): fix getEnv stub and Dockerfile Go version for deployment readiness

The getEnv() function in server.go always returned the default value,
preventing ACB_R2_ENDPOINT/ACB_B2_ENDPOINT from being read at runtime.
Also updated Dockerfile from golang:1.24 to golang:1.25 to match go.mod.

K8s manifests for acb-evolver and acb-api already exist in
declarative-config/k8s/iad-acb/ai-code-battle/ (added Apr 21).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

cmd/acb-api/Dockerfile

@@ -3,7 +3,7 @@
 # and Glicko-2 rating updates. Connects to PostgreSQL and Valkey.
 
 # Build stage
-FROM golang:1.24-alpine AS builder
+FROM golang:1.25-alpine AS builder
 
 WORKDIR /build
 

cmd/acb-api/server.go

@@ -8,6 +8,7 @@ import (
 	"io"
 	"log"
 	"net/http"
+	"os"
 	"strconv"
 	"strings"
 	"time"
@@ -1137,9 +1138,9 @@ func (s *Server) authenticateWorker(r *http.Request) bool {
 	return parts[1] == s.cfg.WorkerAPIKey
 }
 
-// getEnv gets an environment variable with a default value
 func getEnv(key, defaultValue string) string {
-	// This function is a simple helper - in production use the one from config.go
-	// For now, inline the logic
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
 	return defaultValue
 }